Cylinder 4 missfire and lean code

I am working on a 1997 mazda b2300 (ranger Clone).. it has the 2.3 SOHC ford engine.. i am getting 2 different codes.. P0304 and P0171 one is cylinder four missfire and the other is System too lean bank 1... I have replace the spark plugs and wires sence they were origanal and should have been done a long time ago... now i am still geting the 2 same codes... i have checked around for any vaccume leaks and to see if anything was disconnected.. but everything looks good.. also the fuel filter has been changed not too long ago so i know thats good.. checked the coil packs and i am geting good spark... any ideas???

Use a can of carburetor cleaner and spray around the intake and injector where it is pushed into the intake. If you find a vacuum the engine should smooth out.
Look for a crack around the oxygen sensors any where air can get into the system.