Not starting after replacing plugs and leads

Try the HT lead on that cylinder by starting car in the dark and look out for spark coming out of that lead while running. Or get a spare lead and replace on that cylinder.hope this helps

Cold starting misfires can often be caused by either tired high tension spark plug leads or ignition coil packs. If replacing leads, then replace with a full, new set. Coils can be changed individually, but it is often the case that if one has failed, then the others aren't far behind. Some cars have individual coil packs on each spark plug. Some cars have a single pack for all plugs and some have coil packs which may control two or more plugs.

To check for a misfire (ignition failure) follow these steps:

(Cold start) (Use a pair of insulated pliers)

1) Start the vehicle

2) When the vehicle starts misfiring, disconnect plug leads or coil pack connections one at a time. If the vehicle idles even more roughly, then that plug or pack should be considered fine.
Replug each connection before disconnecting the next one. Once you disconnect a plug lead or coil pack connection and find no difference in poor engine running, then you have likely found the offending cylinder. Check the spark plug condition, lead and/or coil condition and connections on that cylinder.

3) Replace components as necessary.

have a compression test done to determine if valve or other chamber problem. change coil to another cylinder if individual coils to see if misfire moves . Change injector to another cylinder to see if misfire moves . IF it has HT leads then replace that lead with a new one and /or change that plug only too see if it is a plug problem. Check inside distributor cap for crack at that plug lead point.

having PO 304 = Misfire cylinder 4,PO 306 = Misfire cylinder 6, as I read above you have done the most. The thing that you have missed is to check whether the Catalytic Converter is faulty or not. To get an idea check whether you have a smell like rotten eggs at the end of the exhaust,you start having problems with cat and needs replacement. My advice before you go for a cat replacement is good to visit an MOT center and check your emissions and then you decide,

first code means random/multiple misfire cylinder (s) detected
second code means misfire detected number 1 cylinder, third code, miss=fire number 3 cylinder fourth code misfire number 4 cylinder
causes can be spark plugs, HT leads, ignition coils, injector, wiring fault, low compression/ engine mechanical fault

Check number four spark plug. The gap could be off causing it to mis-fire.

replace crankshaft senor, its bad, faulty. Check your O2 sensor. change fuel fuel filter and spark plugs and wires with new ones.

check fuel filter and check fuel for water or contaminents
then check fuel pressure with gauge also while driving
may have pin hole in leads - spray water with salt solution over HT leads while engine running and then rev to see or here spark jump(missfire)
recheck spark plugs - did you crack any on installation/removal/installation?
what are the codes and description if stored?
what is engine live data parameters like do they look normal (right specs for range/operation)?

have you changed the ignition wires? perhaps firing order crossed? if not coil pack or ignition wires faulty how do you know it is not "FIREING" on 1 & 4 have you tested it with a spark tester tool???

Do you feel a misfire?? Random misfires are usually caused by a lean fuel mixture, the reasons that cause this a to many to list here.