single cylinder misfire

I've been getting a misfire on cylinder 4 for quite awhile now. It only happens under load. Before I'd really have to push it before it misses, but now it'll miss any time I'm over like 70mph or have to punch it getting on the freeway. I've changed the plugs, wires, caps, rotors, and fuel filter. I've also notice that while its missing and afterward there is a smell of exhaust in the cab. I dont think I have an exhaust leak or anything, cant hear one at least. This thing is really pissing me off. Any ideas? I drive a 99 4x4 tahoe with the 5.7 vortec.

It sounds like you may have a valve on cylinder #4 sticking. You can either rebuild/replace the worn components in your head or possibly blow out the stuck valves with compressed air.

Any other codes?
How is the wire and plug to no 4 cylinder?
What is the compression reading?
What is the mileage on engine?
How is fuel pressure?
Also check the camshaft retard setting with a good engine capable scanner.

There are no other codes, the wire and plus is fine, the engine has 144xxx miles and i havent checked the fuel pressure, compression, or checked the camshaft retard setting.

Check it all out.
Also on plug and wire I like to switch with another cylinder to be sure on them.
What is the mileage on engine and tune up end?

The engine has 144k also. I did the tune up at about 135k. The misfire was there before and after the tuneup, so I know its not the plug, wire, cap, or rotor. I recently changed the fuel filter too. If the problem is a stuck exhaust valve, do you think seafoaming the top end wouldd possibly unstick it. I've heard good things about it, but no horror stories.

Switch plug and wire to be sure.
Even new can be bad.
Make sure you have good spark to plug.
A compression check will show a bad valve.
Check fuel pressure and camshaft retard to.
Post back findings.