For 4e levels 10 or below, (5e level) = 1/2 (4e level).
For 4e levels 11-25, (5e level) = (4e level) - 5.
I don't believe there is any 5e equivalent for 4e levels 26-30.
The reason I use this conversion instead of the "official" one is that the availability of utility spells is a better barometer of whether a character is well matched to an adventure, and this method matches the acquisition of utility spells. Combat ability is only relevant in relation to the power level of the monsters, which are not consistent across editions anyway, and which can be addressed by tweaking monster stats if need be. But when you can levitate, or fly, or breathe underwater can really change how a module plays.