Both 4-outlet and 3-outlet master cylinder brake systems on the Renault 4 are split systems - one circuit operates the front brakes, the other the rear ones. Difference lies in the method used for failure warning: four outlet master cylinders have the PDI (pressure drop indicator) warning, that is, a piston between two pairs of fluid outlets moving when fluid pressure on one circuit drops (=failure), closing an electrical contact. Three outlet master cylinders got rid of the often troublesome hydraulic failure warning system, having a simple float in the reservoir cap that senses low fluid level.
So it should not be an issue functionality- and safety- wise fitting the four outlet master cylinder on the three outlet system, provided that front brake pipes are fitted on the same side of the PDI unit (both front or both rear outlets). I have successfully done the opposite in the past, I fitted a three outlet master cylinder on a 4-outlet system.
Two more notes: 4 outlet master cylinders have a bleed nipple at the PDI unit, this must be bled after all four wheels have been done.
Four outlet master cylinders are usually (but not always) supplied in 20,64mm inner diameter, while three outlet ones are 19,05 inner dia. Depending on your former fitment, you may end up with slightly less braking power or slightly longer pedal travel.