Pontiac aztek 2003, what is my issue?

Hi againagain!  I took my pontiac aztek 2003 to the shop
for a misfire in cylinder 4, code P0304.  A complete tune up
got done,  also the serpatine belt was replaced due to it
being damaged.  The vehicle was picked up with the same
issue starts up, but a little rough to start,  and the engine is
losing power,  causing it to cut off.   Vibration is coming
from the front,  at one point the fuel gauge kept changing,  
it went from saying low fuel,  to being at a quarter tank.  I
was able to start the vehicle and make it home  having to
restart it the few time it cut off inbetween.   What is going
on knowledgeable Aztek Drivers.   Any idea as to what the
issue is?