93 nissan hardbody (emergency)

[nickster050]

My hardbody was running fine but now when i give it gas it boggs down and sounds like it wants to backfire. I can only get it up to 40 mph! I did a tune up, changed wires and plugs, even pulled a pipe and checked the catalytic converter. it still runs bad. when i pilled the plugs all of them looked normal except for the number 4. It was badly burnt. Any suggestions? By the way im on a very tight budget. Thanks in advance.

[pejsa s-13]

you might want to start checking things like your timing, fuel filter, and if possible compression.