You don’t surge on a 4 gem match, ever. This was a “bug” that was in the game from release on PC/Mobile. They fixed this when they ported the PC/Mobile version to Unity, allowing match 4s to surge the same chance that match 3s can. However, fixing this bug extended turns further, which, combined with the pRNG of the new engine and general power creep, created frequent “streaky” situations that players found undesirable. Thus, due to player feedback, the “bug” was patched back in the next major revision, to all platforms, as a “feature”. Due to power creep and new mechanics like doomskulls, the potency of a turn now is actually higher than it was with match 4 surges turned on, but I guess it was more palletable since it happened over a period of time and not all at once.
tl;dr: It is intended that you don’t ever get a mana surge from a 4 gem match. You still always surge on 5+ matches. Your surge chance for 3 matches depends on your mastery.