Using IFNA to check matches in multiple columns

I am trying to adapt a current formula (IF(ISNA(VLOOKUP(A4,S:S,1,FALSE)),(A4&" "&C4),(VLOOKUP(A4,S:S,1,FALSE))) that checks if the number in Cell A4 matches anything in column S, the cell populates with the data in Cell A4. If not in column S, it will populate with both items in column A and C. I need to know if I can add a third paramenter. I have data in column T. I want the formula to first check column S, and if a match to an item there, populate with B4. If not, THEN check column T, and if it matches something in that one, then return amt in C4, but if in neither, THEN retrurn amt in D4. I am sure this is possible, but cannot figure it out...