Sounds like a ignition, coil pack spark plug issue. It kinda sounds like the middle two plugs are grounded out. Before you start buying parts, take the to coils and plugs from the two cylinders that you know are firing and switch them with the 2 non firing cylinders. If it's a plug coil issue the two good cylinders will stop firing and the two bad will start working.

If that dosn't help shed some light start looking for a bad electrical connection, while the connectors may look good you'll need to take them apart inspect and clean them, and not just the plug and ignition connectors, you should check all related connectors in the engine bay. Also these engines can be very particular as to which spark plug you use. Some non OEM brands simply don't work well I always used OEM plugs from the dealer same with the coil packs.

zthis problem sounds very familiar, someone else had a simular problem in the past, so with some luck they will chime in. There is a nice sticky thread about changing the spark plugs that may help shed some light on the issue.

Good Luck