Great shots, and that 90mm looks like it's working great.
Only i have a small doubt about getting a 4:1 with extention tubes.
Suppose you focus at 30mm distance to get 1:1, then you would need 80mm extention tubes to focus at 10mm distance and get an 4:1. What is darn close for a fly.
The only way to really check this is to messure it (with at picture of a ruler or something like that)
I heard speaking about cropping to get an extra magnification but that is to difficult for met to calc.
Any way, like your macro shots.