4℉.
What we know about Degrees is that there is a Positive type and a negative type.
(i.e: 30℉ is positive and -30℉ is negative.)
If the temperature was -4℉ at 7AM, then it is negative. If it goes up by an amount that is more than 4 then that negative will go up to a positive temperature. In this case: At 9AM it was 8° warmer.
Warmer is a keyword. If it is warmer by an amount, Negative temperatures will go up to a positive and positive temperature will just go up. If it gets cooler, negative temperatures will go down further and positive temperatures will go down to a negative.
So lets work out this problem with our newfound knowledge.
-4° F at 7AM
8° warmer at 9AM
-4 + 8 = 4.
The temperature was 4° at 9AM.
-Snooky