Just because someone is uneducated does not mean they are a troll. He could just simply be new to fishkeeping. Since he said 75 gallons you could keep some species of cichlids in there. I can't really think of something that looks like a "Proper shark". You could make your fish really happy and get a smaller cichlid like a convict or firemouth in a 75 gallon easily.This has to be a troll.
You can't keep any LARGE fish in a 4ft tank unless it's something like 4'x 4'. If you want to keep a pleco WITH something else I wouldn't feel comfortable going larger than about 12" max. If the fish gets larger than about 12" don't bother trying to keep it in a 4' tank. Use google, it's your friend.
are red tail catfish at 18" be to big, on its own in a 75 gallon, 4ft? with a pleco or not?