my 1999 bmw 528i check engine light keeps coming on. Was a cylinder 4 misfire. replaced wires coils and spark plugs. 2 days later

Next would be to check for possible fuel injector issue. On that cylinder.

http://www.obd-codes.com/p0304 -- check this out for possible causes and solutions. Also make sure all wire harnesses (coil and injector) are plugged in properly and have no frayed wires.

Did you replace the plugs with the OEM NGK's? I did not not on mine and developed a "lean misfire" problem, having replaced my coils as well, that was remedied by going back to the NGK's

Get yourself a peake research scan/reset tool: http://www.peakeresearch.com/fcx3.htm You will likely need the one for under the hood connectivity. It will tell you what is wrong versus just throwing money at replacing parts which may or may not fix the issue(s). Good luck.