I had a few MBX4's - from the standard, RR, and XR.
They dont seem to soak up the whoops as well as an MBX5 - that was the first thing I noticed....the MBX5 just flattens out whoops full throttle w/o bucking....where as the 4's were upsidedown.

I disagree about parts avail - there are usually a lot on eBay....