Even without a compression tester you should be able to get a good idea of whether or not it has good compression on the two cylinders that aren't firing. Disconnect the crankshaft sensor to prevent the engine from starting and then crank it over. If it's obvious that it's cranking unevenly then you should suspect a loss of compression on the two adjacent cylinders, most likely caused by a failed head gasket.

If the comprssion is ok then next thing to do is to reconnect the crankshaft sensor and check for an HT spark on cylinders 1 & 4, and also on 2 & 3. The ignition coil is actually two coils in one pack. One side fires cylinders 1 & 4 and the other side fires cylinders 2 & 3. As has already been said these pairs fire together, known as wasted spark. If you have no spark on 2 & 3 then locate the plug on the side of the coil. The centre terminal is a common 12 volt supply to both sides of the coil. Theres's not much point in checking for this because you know the engine does run, and therefore this power supply must be present. The outer two wires are the ECM negative return to switch each coil. If one side is switching and the other side isn't then you need to check the wiring back to the ECM, or the ECM has a failed transistor inside and will need to be replaced. It will not be the first time one of these ECMs has failed in this way if it has.