Hi cnt_2013,
If your RA income goes under T4A, then CIC will still accept it, provided that your reference letter follows the guideline provided by the instruction guide (clearly document the money that goes under RA, the number of hours, etc).
I am a living proof of this, and I have also helped several other people from my own department get PR using just TA/RA experience alone, and they all successfully become PRs. Just make sure that your reference letters include everything that is required by CIC (including the duties, responsibilities and lead statements of NOC 4012), and everything will be fine.
Keep in mind though that this would NOT work for CEC, as CIC is being explicit in that they won't accept anymore work experience that is gained while the applicant is studying full-time (co-op, TA/RA, etc).
EDIT:
Parkinson pointed out a very informative link, which was relatively recent, but it consolidates our position all along in that CIC indeed accepts TA/RA as valid work experience, provided that it satisfies all of the other criteria.
As I mentioned, it boils down to whether or not your reference letter mentions duties (AND LEAD STATEMENT) that are substantially matching those of NOC 4012, and whether or not you can get at least 1,560 continuous hours (if you rely on these hours to satisfy the minimum work experience requirements). There is no mention whatsoever that the income needs to be taxable.