This was a tricky one to tie up all the loose ends, but I think I finally have it...
Each of the connecting groups exhibits:
a different type of wordplay before you can find the answers.
Group 1: DEALT, HIP, MAGMA, TEA
Anagrams of Greek letters: DELTA, PHI, GAMMA, ETA
Group 2: BURN, FELT, LAND, WOOD
Words that can follow 'HEART' to make a new word: Heartburn, Heartfelt, Heartland, Heartwood
Group 3: ARROW, MAGNET, PROFESSOR, SAG
X-Men characters missing a letter: (M)ARROW, MAGNET(O), PROFESSOR (X), SAG(E)
Group 4: ATONE, DISTRESS, POKER, SOY
One-word Robert De Niro films with the first letter changed: (S)tone, (M)istress, (J)oker, (J)oy
And the 4-letter word that can fit in all these categories? I told you already!
- An anagram of BETA;
- Follows 'HEART' to make 'HEARTBEAT';
- Add an 'S' to make the X-Men character BEA(S)T;
- Change the first letter to 'H' to make the Robert De Niro film, Heat