Well, never kept a gar, and just some looking into the gar section I found the following.
Gar Tankmates - as of right now only people who have posted keeping it with cichlids have kept them with a Jag or a Black Diamond.
Gar Tank Size - For an 18" gar "nothing under 5' long and 30" wide is advisable", I don't know if there are smaller gars or not as I've never really looked into them.
**Edit #1**
Smallest Gar I could find in the gar info stickies is the
Flordia Gar and it gets (according to the link) 18-22".
So based off of that info I don't think you could house for life a gar in your tank.
**Edit #2**
A 4' tank doesn't give CA/SA cichlids much room to establish territories which is needed. I know there are some people here who over stock their tanks so the aggression is spread around. Because of the footprint what about your 2 Oscars, some clown loaches then some type of dither/target fish?