Yes, as stated, waaaaaaaaaaaaaaaaaaayyyy too small for four oscars. this tank is big enough for one full grown adult oscar and not much else. If you want to keep two adult oscars, a good starting point is 180G. This will allow them each to set up their own territories and if enough decor is placed in the middle of the tank, the o's may never have to see eachother. Also, in the 180, you could have a school of 6-8 SD's pretty comfortably and would give a dither fish for the o's to feel more comfortable and maybe stem the aggression.
Also, if the lfs that sold these fish to you (and it was one big purchase) didnt ask you how big your tank was, then don't ever go to that lfs again unless that is your only option as they don't care about the fish or your fishkeeping experiece, only about taking your money
I guess I should mention for the other fish too:
- balas are schooling so need a group of atleast 5-6 so need 120G for them
- 2 plecos (most likely common plecos) will grow to be huge (possibly 20" + when full grown) so will outgrow that tank
- not sure on the scats and it would help to know what type of eel it is as well. The smallest species, peacock eels, still grow to 12 to 14" easily
You've got some potential "Tankbusters" there so either upgrade the tank and remove atleast two of the o's are your gonna have big problems when all these fish start reaching mature size in a year or two
welcome to the forum 
