That is going to be an interesting task. If I were you, I would make those 4 injectors run in open loop. No feedback from O2 sensor to those particular injectors. Let the port injectors run in closed loop.
You could drive those 4 extra injectors with a stand-alone unit of some sort. It doesn’t have to be complex or expensive unless you want plug-and-play products that are found in the women’s section of the automotive store. Get yourself an Arduino. Let that Arduino read throttle position. Use that throttle position voltage to dictate those 4 injector’s pulse width and frequency.
If you don’t know how to program, this Arduino project will fix that. If you aren’t great with electronics, this will fix that problem too. If you don't want to get good at that stuff, you should've just slapped a pair of carbs on top of that 671 yesterday.
"Formal education is a weapon, whose effect depends on who holds it in his hands and at whom it is aimed." ~ Joseph Stalin