Start with checking what you think it is, pull the fuel filter and see if you can get fuel to flow freely through it.
If you think two of your cylinders aren't firing, run the engine and pull the spark plug wires off one at a time, listen to see if the sound of the engine changes at all. If it chugs or dies, that cylinder is firing. If it doesn't change, the cylinder isn't firing. Put each wire back after you've tested its cylinder. This will help you identify which cylinders are misfiring.
Once you've done that, I would stop the engine and pull the spark plugs out of those cylinders that are missing (and disconnect all 4 plug wires). If the plugs are covered in black soot, whitish powder, or really anything other than tan, make a note of that. Are they covered in fuel or dry? If they have fuel on them then wipe it off, then test them for spark by touching the threads to the engine block, with the plug in its respective wire, and having someone turn the engine over. If you see a strong blue spark, it's good. Otherwise look closer at your ignition system.
If you have good spark and there is fuel on the plugs, get a compression tester like this one here and check the compression on those cylinders by threading the tester into each of the bad cylinders at a time, with the plug wires off the other cylinders, and have someone turn over the engine. It should read at minimum 100-120psi. A good way to confirm the reading is what it should be is to compare it to one of the known good cylinders.
If you don't have the correct level of compression, try adding a small amount of oil to the cylinder and testing again. If the reading goes up, the piston rings are worn and the engine will need to be taken apart to replace them. If it doesn't, it's very possible they skipped a tooth when installing the timing belt at the shop and some of your valves are out of time, but could also mean the valves are damaged.
If you don't have good spark and you know the plugs, wires, and distributor cap have been replaced, I'd look at your ignition coils next.
If you don't have fuel, I would check to ensure the new injectors are correctly connected (double check the electrical connectors for them, and if that looks good then pull them out to make sure their o-rings aren't pinched in front of the nozzles), and that they are flowing fuel. Checking for fuel flow can be a more involved task, but you can find write-ups on how to do it online.
Hope it helps!