Hi sunnysmom. Guilt is not a constructive emotion but it plagues all of us as mothers!! It's as if whatever we do, we feel guilty in case the other option was better! I have a friend who outright resisted giving formula but then felt really guilty when baby dropped a few centiles on the weight chart and it was discovered she did have a genuine supply problem (very rare, by the way). So of course she feels guilty about her decision! Know that you did your best at a very difficult time. We don't have to do the absolute best or be perfect in order to be doing well, IYKWIM.
If you were very determined to get off the formula, you could start expressing in the evenings to re-build your evening supply. It probably wouldn't take more than a week or so of expressing and feeding before your supply was up again, since you're still doing other breastfeeds. Express for a couple of days, then breastfeed baby instead and top-up with bottle whilst you express to stimulate the breasts more (even if nothing comes out, you're signalling to your body to produce more milk).
Having said that, your baby is getting marvellous benefits from the breastmilk that you are providing and you've done great to breastfeed at all to 4 months. You needn't put a black-mark on the great work you've done by feeling guilty.