As the middle two are different than the top and bottom ones, it's very unlikely to be anything that's common to the engine (fuel line, tank, computer, timing, etc.) but rather, individual to them.
Look at plug caps & leads, coil packs, etc. and follow it back, checking, until it becomes common. You can check the spark with an adjustable spark tester as an indication to see how far each leads spark jumps. Although this is a rough and ready check, if one or two cylinders are vastly different, it can help point you in the right direction.
Same with the fuel - check wiring, connections, etc. on each injector and trace back.
Also, having different compressions on each cylinder will cause each plug to look slightly different, so check them. It's worth doing them twice - once with all the plugs out, and again with all the plugs in, apart from the one you are checking obviously. If they are different between checks, that can give an indication of leaking head gasket.
More serious problems like worn camshafts, pitted valve seats, etc. would be way down my list, unless it's done an inordinate amount of hours and never serviced.
