Will a 4 series rear work in place of a 3 series? My buddy thinks the only difference is my ABS light will come on.....Guess what...it already is...so thats not an issue....So basically I need to know if it will cause any malfunctions in the computer, loss of speedo? Anything come to mind?

 

Not at all.By series i assume that you are talking about "channel".

I ve done it but in the other way.I ditch my 4 channel stock 10 bolts for a 3 channel 12 bolts.

The only difference between the two are only that the 4 channel come with two reluctor ring,one at each end of the axle and also 2 abs sensor.

Thats it.No malfunction,no loss of speedo(if you keep same gear).