what are the differences in 4cyl and 5oh t5's?

the 4cyl input shaft is smaller and when using it with a V8, requires a pilot bearing from a saab. The V8 trans has the larger input shaft and can be turned down in order to be used with a 4cyl.
The torque rating on the V8 is slightly higher but I wouldn't try to "build" to the strength of a V8 trans. I had a 4cyl trans behind a 351w that ran 7.8 in the 1/8th. They will take a lot of power but none of them can handle loads of wheel hop or missed gears.

Several companies sell rebuild kits to make your own Z spec trans, which is probably a good alternative.

V-8 vs. 4-cyl T-5's - Differences:

1. Gear Ratios - The 4cyl tranny has a lot closer ratios, and the 5th gear isn't really much of an extra gear compared to the V-8 tranny.

2. Input Torque rating - The 4-cyl tranny has a slightly lower input torque rating than the V-8, but can still handle V-8 power, look at the Turbocoupe and SVO trannies.

3. Input pilot diameter - a special pilot is required for using a 4 cyl tranny behind a V-8.

the casing, syncro's, blocker rings, thrust bearing, and main shaft are the same, with the exception of the input bearing retainer, the V-8 trannies use a larger bearing and the retainers are not interchangeable.

Having a 4-cyl tranny behind a V-8 is like having gears out back, my bud has one in his mild V-8 and it's like he has 3.73's.

Having a V-8 tranny behind a 4-cyl is like have 1.73 or so gears out back, it's too steep of a ratio and keeps the engine dogged out all the time, unless you have 4.10's or better out back.

As mentioned, there are "z" spec kits out there to upgrade the T-5, although i don't know if they make them for the 4cyl trannies.