I've tried Ukulele and KeyRemap4MacBook. Neither does what I need to do.
I want to map the top-row 4 key and the top-row 8 key to SHIFT. So that, for example, when I press 4 + g it types a capital G.
Hitting the shift key with my pinkie is aggravating my carpal tunnel. On my windows machine I just remapped 4 and 8 to shift and I use a keypad if I need numbers. Savior for my hands.