You can go T12, T8 or LED, and 2, 3 or 4 lamps. That is decided by which ballast you buy.
Instant, or Rapid/Programmed Start.
This is decided by the wiring of your fixture. Each bulb end has 2 pins because there's a small filament in each end of the bulb. Rapid or Programmed Start ballasts use both pins to preheat those filaments for an easy start and longer lamp life - so they have 2 wires to each end of each bulb. Instant start ballasts do not, so they only have 1 wire. If you have instant-start, stay with it - otherwise you may need to replace the lamp sockets.
T12 vs T8 vs LED
You probably have inventory of T12 bulbs, but they are basically obsolete. T8 bulbs are inexpensive, more efficient, have much better color rendition (typically starting at 80% CRI) and come in several light colors (3500K, 5100K etc.) Edit: since I wrote this I have started fitting 90CRI T8 tubes, readily available for under $2 each, they are marvelous. You don't see it in the tubes, but rather, in the stuff being lit up.
LEDs come in 2 flavors: "direct wire" (rewire the fixture for 120V on the lamp ends) or "keep the ballast". (no rewiring, the smarter LED emulates a bulb). The trouble with the second kind is you must maintain the ballast, and it must be of a certain type (i.e. instant start). With direct wire, you remove the ballast entirely and wire 120V to the lamp end(s). Some "direct wire" LEDs take the 120V on one end of the lamp (the other end is a dummy) - that can be a problem in a fixture with 1-wire sockets.
2, 3 or 4 bulbs
T8 bulbs are generally brighter than T12 bulbs. Look at the bulb rating in lumens, but multiply by the ballast factor (a percentage) - they make ballasts designed to underdrive or even overdrive the bulb's rating. Often when lighting is carefully designed, they replace 4 bulbs with 3 or 2, because T8's are brighter and/or because they found the space was "over-lit".
LED lumens are lower, but they're worth a lot more, because they're all aimed in the right direction - unlike fluorescents which waste a lot of energy lighting the backside of a reflector.
Sourcing
Changing ballasts is rather straightforward and I pay between $12 and $30 on 1000bulbs.com or other sources. I aim for middle to top shelf quality - my first choice is GE, and I find good deals on boxes of new ballasts on eBay. Edit: 4-lamp ballasts are readily available. The tactic of getting 3-lamp ballasts and only fitting 3 tubes works well with T8, as they are somewhat brighter.
I have also been doing great on eBay buying new or lightly used ballasts that were pulled from newish fixtures for LED upgrades. As low as $4 each.
I regularly see direct-wire LED replacement 4 foot "tubes" for under $10, often from 1000bulbs.com or superbrightLEDs.com.
It's pretty easy work if you don't mess with the lamp sockets - just get a big bag of "blue" wire-nuts, a stripper, and go to town. If your fixture is hard-wired, you must install "disconnecting means" - I power down, install half the connector on the hot lines, then power up and do everything else.
When I replace lamp socket ends ("tombstones"), I pay about 60-70 cents each for those, and they are fairly standard. But the labor gets pretty high.
You're much better off with the (did you notice, much lighter) electronic ballast. Instant start is fine, since it's the only type possible with 1-pin ends.
There are only 4 parts to a fluorescent lamp: Chassis, lampholders, ballast and lamps. You just eliminated ballast. Lamps, I'm trusting you here. That leaves lampholders.
Look at the lampholders closely, make sure they are intact and the jumper between the yellow lampholders is intact. 8' 1-pin replacement lampholders can be had for a couple of bucks, I replace them all the time. (4' 2-pin lampholders are like 60 cents.)
Best Answer
Those aren't strip lights; that's something else.
If you like your old fixtures, or don't want to deal with the ugliness of a swap, keep them. Every part of a fluorescent fixture is serviceable. If the ballasts are fried, you can just do a ballast swap. You can also convert to LED. All within the old fixture.
The fixture is the expensive part, so swapping ballast or going LED will be the cheapest repair, and lets you keep the quality fixture instead of dealing with the absolute junk fixtures I see in big-box these days at the $20-30 price point.
If you want to switch to T8, the T12 and T8 tubes are physically compatible, so T8 tubes will fit in a T12 fixture. They are not electrically compatible, so you must change the ballast to T8.
You can also get LED "tubes" that snap into T12 or T8 sockets. Get ballast-bypass (direct wire) types which are fed from opposite ends of the tube. Then rewire your fixture to bypass the ballast.
The 4' and 2' fixtures will be compatible, they share only 120v mains power and have separate ballasts.