First off,I would not let the dealer do anything just yet....With that being said,I personally would wait until the CEL (check engine light) came back on,and read the code.You might also want to consider purchasing an OBDII reader,and keep it handy. These will work on almost all makes/models,and are pretty much a "must" for todays newer vehicles..............................Now back to the issue...Since the plugs,and wires have been changed recently,I have doubts that its either causing the issue.There is of course the possibility that either the plug on #4 is bad,or the wire.Wires can be damaged during installation,or removal.Plugs can get a bit of "garbage" on them,and misfire.....anythings possible.