I believe the 89 IFS is the same as the 86 IFS.

My 86 stock aluminum rims are 15x7 with 15mm offset = 4.59" backspacing....

A 15x8 rim with 4.59" backspacing would be 2.3mm offset.   I'd just go with 0 offset and 4.5" backspacing.

If you are going to run 4" flared fenders w/an 8" rim you don't want a back spacing of 4.5, or anywhere near 4" for that matter .  The higher the BS, the closer in your wheels will be.  With a 4" back spacing on an 8" rim, your tires would sit 2" or more inside the fenders, and would look rather funny.   You are going to want about a 2" backspacing, and then it is likely the tires will still sit further in then the 4" flared fenders. 

If you are buying new rims, you might want to opt for 10's and not 8's.  I will post up up pics of my '90 w/3" Warn fender flares and 15x10's w/a 3" BS(IIRC).  The 33x12.50 tire are just at the edge.  I will also take some track width measurements of the '90 and my '91 that has 35x12.50 on 15x10's w/2" BS.