So first off is there any real difference between an s4 diff to the s5???
Also would the L.S.D be interchangeable between the two if there is or isn't any difference between them???

any answers to these would be greatly helpful in my better understanding my car a little bit more so that I could be an informed tuner.

Thanks I appreciate any given responses

 

S4 LSDs are clutch-type, S5 LSDs are viscous. You can't swap an NA diff with a TII diff, but you can swap the same kind between series (ie S4 NA into S5 NA). All TIIs had LSDs, but some NAs had open diffs. You can check the standard specs sticky for dealer brochures that list what model had what.