Converting 2 lamp ballast to a one lamp

I'm a bit alarmed when I hear you say "capped off my yellow wires". A few ballasts do give that instruction, but not for 2-bulb use. I fear you might be matching up wire colors and not following the schematic on the ballast. Thing is, if the old ballast was instant-start, that will totally backfire. As you can see in these typical schematics, red and blue do quite different things on the two types of ballast. (ignore the LED one).

These schematics are not universal, only common. Yours is on the label on the ballast.

When converting instant-start to rapid-start, the other issue is shorting . The instant-starts only connect 1 wire to each end of the bulb, but it goes to both pins. This is almost always accomplished with a shorting lampholder (tombstone), which accepts one wire and connects it to both pins.

A shorting lampholder will have 2 holes for wires, but that's for daisy-chaining (as in the red wire above), not for 2 separate wires. A non-shorting lampholder intended for rapid-start ballasts will have 4 wire holes, 2 for each pin. The extras are for daisy-chaining, as in the yellow below.

Unless there's a visible wire to remove, there is no way to turn a shorting lampholder into a non-shorting one. You just need to buy replacements. They are fairly standardized, and I buy mine online for about 60 cents a tombstone. I buy only non-shorting types, it's easy enough to convert one into shorting with a wire.

Ballast replacement is fairly straightforward. I still prefer fluorescent tubes because I can easily find quality fluorescent tubes in 90+ CRI (in fact Menards stocks them), and I have serious doubts about the real quality and longevity of the LED "cheapie" tubes.

Start with a "ballast disconnect"

For this you need to shut off the circuit breaker supplying power. You then put the ballast disconnect at the point where the power supply (typically 12-14 AWG wire) meets the ballast (18 AWG wire). Some disconnects care which side is the heavier wire, i.e. the yellow Wago types. If there are 2 power cables going in and out, then either use the Wagos (which provide for that) or pigtail it.

Once that's done, you'll be able to service the fixture without turning the breaker off. Just unplug.

Next select the bulbs

Those look like 48" bi-pin tubes (the most common one). From looking at the "old" Keystone ballast, your old ballast was for F32T8 tubes, 1" diameter (T8). Which means it's not that old. Don't use F40T12 tubes, 1-1/2" diameter, they will fit but they are not electrically compatible unless you change to a T12 ballast.

Look at the existing ballast

The big label on the old Keystone ballast says your bulbs are instant-start, with 1 wire per end of the lamp. Gotta watch out for that 2-yellow 2-red 2-blue color scheme as it's also used by 2-lamp rapid-start ballasts. This is an example where the diagram really matters.

Check the new ballast also, the wiring may be subtly different. If the Lowes clerk picked your matching ballast by the same wiring diagram, that's handy, but double-check the type of tube it's made for. Here are generic drawings of ballast wiring; yours may vary. For instance yours wires like two separate 2-tube instant-start ballasts.

This is from my big answer here.

Historically 4-lamp fixtures often had 2 ballasts. There is a mounting site in your fixture for a second ballast. Aside from using the more common 2-lamp ballast. it also let you feed them from 2 switches for "high" and "low".

If the ballast is a goner, you'll have no trouble finding 4-lamp ballasts in either instant (1 wire) or rapid-start (2 wires). Rapid-start gives better tube life; a sub-type is programmed-start which gives superb tube life - something to think about if the fixture is hard to access. But if you switch to rapid-start, you'll need to change the lampholders (tombstones) on the end of the tubes to a non-shorting type.

Edited significantly based on updated info