You’ve converted the absolute value, not the delta.

The quickest in your head conversion of a delta °C --> delta °F is to simply multiply by 2. +4°C --> +8°F

To be accurate, remove 1/10^th of the °C value before the multiplication. +4°C --> (4 - 0.4) = 3.6 —> +7.2 °F

For absolute values, I personally do, for a quick ‘in your head’ approximation the other way round (°F --> °C) :

1. remove 32 °F (78°F --> 46)
2. divide by 2 (46 --> 23°C; I may stop there for a very rough approximation)
3. add 1°C per decade (if you got 23°C, add 2°C --> 25°C; I usually stop there)
4. add 0.1°C per unit (25 --> 25.5°C; it’s not the exact result but not too far)