I don't know why it wouldn't

S4 V6 vs A4 4 cylinder.

If I remember correctly people who wanted a 6 speed in their A4 1.8T's were putting in S3 trannies.......

Discussions on the B5A4 board took place a few years ago... so my memory is fuzzy

The swap may have involved an S2 6 speed.
S2's were derived from Audi 80/90
You'll have to search.

way for a 6 speed in an B5 A4 1.8t in the Nue S4 trans (2000-2002)with S4 driveshaft and rear diff. You also have to swap the A4 shifter for the S4. Also the drive flanges have to be swapped in both trans and r diff to fit the A4 axles.

Once fully built, is the S4 tip any stronger than an A4 tip?

Which means it will physically bolt in and would normally work but the final drive ratio is differnt. So the only way to make it work would be to swap the rear differential also. Unfortunatly the S4 final drive would not be desirable with the smaller A4 motor.