2011 Chevy Malibu ltz

I just bought this 2011 Chevy Malibu ltz
from a shop about 4 days ago, As is, no
warranty. Well the same night the check
engine light came on, took it to orileys the
next day it says "cylinder 4 misfire" and the
service traction light came on. We changed
the plug in 4, thought maybe it would work.
It didn't. So we switched the coil pack with
#2 and figured if it was that, it would show
"Cylinder 4 misfire". I'm not really sure
what to do. The car lot told me not to
worry that he would help fix it when he get
a mechanic but I'm being told to try and
take the car back since it'll cost a lot to fix
it. It has only been 4 days since I took the
loan out on it. Any suggestions?