Change the management, if its not up to the task you car requires

Although EFI would be better than a fifth injector, its still a very cost effective conversion compared to an engine rebuild.
Once you exceed the fuelling or any other capability's of an ECU you should start to consider an alternative management system rather than try to work around these limitations imho with a fifth injector and similar

I don't personally believe you can do things well with champagne idea's and lemonade budget, if you want a good reliable power you have to be willing to spend the money to obtain this.

champagne idea's and lemonade budgets are the reason there are so many ropey and unreliable modified cars out there imho