In trying to get finer grain with an EI 400 film, use a relatively standard MQ developer such as D-76 or ID-11 either straight or 1:1. A developer that contains phenidone will give slightly more graininess, while otoh it'll give a little more speed and acutance.(posted 8383 days ago)You could of course get significantly finer grain by using Microdol-X or D-25, but your EI 400 film would become EI 200 or slower.
Keep process temperatures moderate, 68F to 73F or so; significantly higher or lower will cause more graininess.
Consider your water supply. Excessively hard water will cause more graininess, even if it's only used for the wash.
As for running C-41, it's easy. Remember, you don't have to be concerned about color shifts; the only thing that'll change with temperature or timing variations is contrast. You can put your tank, developer and blix containers in a plastic basin waterbath kept up around 102F with running hot water, and as things cool off as you agitate the tank etc you'll have a development-end temperature of around 98F for an overall temperature of 100F. The blix goes to completion and doesn't need remotely as much temperature control.
Ilford's XP-1 chemical kit was ordinary C-41 developer used at a dilution of the equivalent of about 1:1 for 5'/100F rather than the standard C-41 3'15"/100F. You can use any other C-41 developer at 1:1 with around 5'/100F as a starting point. The reason XP-1 was used at 1:1 is that a timing error at five minutes was much less significant than a timing error at 3'15" and the process was intended to be used in hand tanks.