Since the 990/4 Mini was based on a TMS9900, Probably
not much.
Dont know if
its enough to run DX10 though.
DX10 requires the memory management of the 990/10 or /12 or a 99000
A simulation of the 990/10 that can run DX10 is present in MESS.
TX990 could run on the 990/4 or /5
http://bitsavers.org/bits/TI/990/TX990_TXDS_2.0/
http://bitsavers.org/pdf/ti/990/txds/