Injector 4 looks like it has had its pintle cap replaced, whereas 1-3 have, probably, their original pintle caps.
The pintle caps are discoloured from use but they don't look like they have carbon buildup on them, per se.
It's not too hard to test EFI injectors.
- Disconnect the positive wire of the coil so you don't have any ignition.
- Re-install the injectors in the fuel rail, re-connect the fuel delivery/return hoses if you disconnected them, but leave the fuel rail out of the manifold.
- Leave the injectors disconnected (electrically)
- Crank engine, check no fuel is dripping out of the injectors (they're not plugged in so they should be closed)
- Grab something like a coke bottle, plug the injectors in one at a time, crank the engine, and observe the spray pattern from the injector, it should be a reasonably consistent fan-shaped mist of fuel. If it is a jet of fuel, or the pattern is wonky or inconsistent with the other injectors, that is a problem.
