I believe that whenever a player gains a 4th crystal it immediately becomes a mana TOKEN.
It's somewhere in the rules but I cannot point you to it.
The "extra crystal becomes a token" rule is in the "Walkthrough" book on page 6 under "Mana". This can occur when a card play gives you a 4th Mana of one color.
However, the example from the OP concerns a combat reward. Since it's a reward from combat and it's gained only at the end of your turn, you get nothing in this case, since tokens don't last from turn to turn. This is on page 9 of the "Main" rulebook, numbers 4 and 5 under "End of the Turn".