1999 Pontiac Bonneville no 4 cylinder misfire

Most commonly a misfire is due to loss of spark, usually from spark plugs and wires, but Ford has a lot of trouble with individual ignition coils and GM has a big problem with mismatched injectors. The mismatched flow rates show up as misfire fault codes at higher mileages.

One way to narrow down if the injectors are the cause is to switch two of them, erase the fault code, then see if the misfire moves to the different cylinder. If it does, replace all of the injectors with a flow-matched set. If you do a search for "Jim Linder's" web site, you can read about the problems GM has been having and how they fix what the manufacturer didn't care to.