There is nothing you can do now but wait . . . at the least until you receive AOR.
After that there are multiple ways you might choose to go forward, such as submitting a supplemental correction AFTER you receive AOR or waiting to the interview and be prepared to correct it then. It will probably be worth a telephone call to the help centre to specifically ask what should you do if you left a trip out of the presence calculation, but you still have enough days to qualify for citizenship.
Nonetheless, what you actually then do, that will need to be a decision you make based on your own personal judgment.
Many applicants make such mistakes. The IRCC website actually encourages applicants to wait longer to apply precisely to be sure they have enough days to meet the presence requirement, allowing for mistakes made in the presence calculation. IRCC is well aware that applicants make such mistakes innocently and is generally very forgiving . . . so long as, of course, it does appear to be an innocent mistake.
Four weeks is probably a big enough omission it is worth sending off, at the least, a supplement explanation to be added to your application, describing the omission, precisely giving the dates of travel (date of exit and date of return to Canada), country or countries visited, reason or purpose of the travel, and an explanation for the omission (whatever the truthful explanation for making the mistake is). It might warrant a revised presence calculation, but probably better to wait for direction from IRCC before doing that.
The risk, of course, is that making such an error naturally raises concerns that there may be other omissions of travel as well, and broader concerns about how reliable your information is generally (an applicant's credibility, after all, is about how much IRCC can rely on the applicant to accurately report the facts, and a significant error in reporting the facts can indicate cause to not rely so much on the applicant's accounting of facts). So a timely acknowledgement of the error and correction, together with an explanation which makes sense, should fairly easily allow IRCC to catalog this as a typical innocent mistake. No harm, no foul.
There is no way to acknowledge and correct the error until you receive AOR.
The reference to AOR leads to a tangent:
I absolutely concur with "Don't panic and just wait."
Beyond that, there are always at least three outcomes (but recognizing there are multiple paths even among these three), not just two:
-- application returned
-- application results in grant of citizenship
-- application is denied
Perhaps ironically, one of the possible outcomes you reference, the possibility the application is returned, has NOTHING to do with the issue raised by the OP's query. An entry error in the physical presence calculation, including the omission of a trip, will not result in an application being returned UNLESS the error on its face invalidates the presence calculation or shows that the applicant did not meet the minimum physical presence requirement.
In particular, failing to enter a whole trip in the presence calculator has NO impact on the completeness check. This kind of error does not result in the application being returned.
By the way, while there are errors for which one can say (and on occasion I probably have said) that the worst case scenario is that the application is returned, that is not nearly so bad as more serious worst case scenarios (such as being accused of making a material misrepresentation), and not nearly so bad, even, as non-routine processing resulting in full blown RQ. That is to say such an error is not all that serious, that the worst case scenario is NOT SO BIG A DEAL. A returned application is nothing to worry much about, so long as what is required to make the application complete can be included in re-submitting the application. Yes, this delays when the oath will be taken. Not by much more than how long it takes to re-submit a complete application.
Sure, all applicants want to get it right and submit an application that is complete, that goes into process, on track for routine processing which leads to taking the oath sooner rather than later. There is no need to fret over mistakes that might render an application incomplete, however, since such mistakes are readily and, ordinarily, easily corrected.
Failing to disclose a whole trip in the presence calculation, however, again, is NOT the kind of error which results in an application being returned.
The other scenario you proffer is possible but the prudent applicant will not idly stand by to see if things turn out this way. The omission of travel totaling 28 days is a substantial omission of fact, even if it does not result in the presence calculation falling short of the minimum required. Odds of IRCC discovering the additional time abroad are substantial. An intentional omission of this magnitude could be very serious. But of course applicants do indeed make such mistakes and IRCC will recognize many such failures to accurately report the facts do indeed happen because of innocent mistakes. But a discrepancy of this magnitude virtually demands clarification, correction, and probably at least enough of an explanation to satisfy IRCC it was a mistake and not a willful omission of material fact.
Which brings up the variable paths to other possible outcomes, namely the various modes of non-routine processing, full blown RQ looming as the more ominous.
In particular, unless there is a timely acknowledgement of the error, a credible correction, and a credible explanation, an omission of this magnitude could easily trigger non-routine processing. At the least this would likely entail a more probing interview and some collateral inquiries to verify the applicant's information otherwise is reliable, in large part verified as accurate. It will likely trigger a closer analysis of travel history, address history, and work history. It could easily trigger full blown RQ for which the applicant will need to submit extensive additional information and documentation, which not only is inconvenient and often results in long delays, and it can be profoundly intrusive into the applicant's privacy, and depending on how much documentation the applicant has to actually prove time in Canada, can put the outcome of the application at risk.
More than a few applicants have reported missing whole trips and not even realizing it until the interview, or not acknowledging it until the interview, and while some incurred small delays, for many their acknowledgement and explanation during the interview has sufficed to satisfy IRCC it was a more or less innocent error. Obviously, this only suffices if there is enough of a margin over the minimum that it is clear they applicant nonetheless met the minimum physical presence requirement.
An omission of nearly a full month (28 days) is big enough, however, probably better to be proactive in correcting it with IRCC rather than waiting until the interview. After AOR of course.