Yeah, I used to work for audi so let me just set the record straight.
A4 2.0T or 3.2 are base cars just like the 328 and 335.
- An s-line is a "sport" package added onto one of the regular A3, A4, A5, A6, A7, or A8 models: includes wheels, body work, suspension, interior upgrades and sometimes a small power bump. (you don't call an M1, M3, and M5 the M-line; you call them M3's or M5's) This is the M-sport Package equivalent.
Also; in the past this has been associated with the end of model life because the last few years of every model are almost all "s-line" cars (2011 3-series mostly had m-sport packages) but it is not only offered those years.
An S3,S4,S5,S6,S7 and S8 are all referred to as separate vehicles from their "A" counterpart as they are now officially "sport" vehicles
- S cars have different body, grills, drivetrain, interior, options, engine. Everything is different except for the basic design on an S4 compared to an A4.
Lastly is the RS models, these are the "M" car competition. Built motors, upgraded drivetrains, more body work, etc.
To finish this, BMW and Audi each chose to position themselves in different niches. Where the BMW goes 328, 335 (optional m-sport package), M3 Audi goes a4 2.0T, 3.2 (optional s-line package), S4 and then RS4.