its neither 4 or 7. I saw one of my friends acquiring CBSA record and he said he got it by default for 14 years( 2000 to 2014). that's strange but I trust his words.
CBSA records are only required if CIC asks for it. When you fill application form, you check the box to authorize CIC to collect records from CBSA on your behalf. If you don't check the box and select No, then it may delay your application and then you might have to send CBSA records.
I don't know why people would select "No", I have absolutely no idea why people will select No. If they do, their application will be delayed and other issues may arise
You don't need them if you said "yes" on your application.
You don't pay anything for them. When I requested them, I specified the time frame(4 years) I was asking them for and that is exactly what they sent me.
Also, it takes up 2-3 months or more for CBSA to process your requests and send the record to you. So IF you plan on getting them, consider the processing time. The reason i asked for them was because I had multiple trips to US during the 4 years before I signed my application. And I did not remember all of them, so I asked for the record just to be safe. Also the record only indicates the date and time you ENTER canada. They do not contain the date you left Canada.