A Master Electrician here to help. I am a double qualified; USA & UK electrician. Please give me a moment to formulate a reply.
Please describe the colors presently in your old fixture and which end they go to.
I mean color of the wires and how many at each end.
That is confusing and you do not need to push anything in you will just need orange wire nuts. I ONLY need to know how many wires are going to each end of the fixture and what colors. Not what is on all the existing ballasts. Please try again, I think it might be as simple as; 2 yellows one side and 2 blues plus two reds on the other side of the fixture ??
If this is the case then just connect one color matched to one color on the single new ballast and you are done. If not the case then please tell me again but ignoring old ballast wiring.
Cut the old wires near to the old ballasts and use orange wirenuts to join them all.
Perfect! and yuor new ballast should have exactly the same count and colors so just cut the old wires near to the old ballasts and using orange wire nuts reconnect old wires to new single ballast.
I presume you meant FOUR tubes ?