I changed the spark plugs and now it's misfiring

The plug wires could. if it a 2.2 if it's a 2.4 bet on the coil cover,
the big plastic piece that houses the coils.

either you have a faulty gas pump, one or more spark plugs or cables ar going bad or nrrd to br replace, could be a vacuum leak too, yourcar have the check engine light on? if so, need to know what code the will help you a lot.

Depending on your Grand Marquis's mileage...it sounds like plug wires and plugs...(worn out) and or engine timing/firing order? If plug wires recently changed...the spark plug wires are in the wrong firing order...confirm the firing order from your local parts store by phone...and retrace each one...always start with #1 cylinder...hope this helps.

Make sure the spark plugs are properly gapped (this will cause misfires). Replace all spark plugs wires!! Insure there is no oil inside the spark plug chamber.

Make sure you didn't cross thread the plug it could cause the engine to run erradictally and not fire correctly

it cant be cross fire it is misfire, cross fire ocurs in engines that use a distributor, the code is from one or more cylinders misfiring check for a bad set of cables or a faulty spark plug or coil

Please describe miss fire? or do you mean it has a miss? cross fire occurs when plugs are firing before or after they are supposed to if it's missing then that's another issue could you please give more detail?

crossfiring/ crossed wiring - check the firing order to ensure proper firing sequence..

have you changed the ignition wires? perhaps firing order crossed? if not coil pack or ignition wires faulty how do you know it is not "FIREING" on 1 & 4 have you tested it with a spark tester tool???