National Express do not run direct to London Victoria from Terminal 4 in the daytime (only overnight at 0327 and 0405).
In the daytime it is an hourly service for a 30 minute ride to Central Bus Station (at Terminals 2 and 3).
Officially you then have a 50 to 60 minute wait at Central Bus Station (CBS) for the connection to Victoria, but in practice most drivers will take you on the next bus after a wait of 15 minutes or less. It ensures a robust connection time.
Saturday departures are at 0510, CBS 0520/0610 Victoria 0710, 0700 CBS 0720/0755 Vic 0850, 0750 CBS 0820/0855 Vic 0950,
then 0855, 0950, 1110, 1200, 1310, 1410, 1600, 1750, 1930 and 2040 (last bus- but there are far, far later buses from CBS, in fact the service runs 24 hours a day).
It very rarely takes the scheduled 55 minutes from CBS to Victoria. I'm well used to that being 35 to 40 minutes even in heavy peak traffic, it's padding against all likely eventualities to ensure adequate driver rest time at Victoria.
The initial bus from Terminal 4 to CBS is to somewhere further afield- like Swansea, Cardiff, Leeds or Derby, maybe other places.