Legal day VFR minimum in Canada is 30 mins reserve, as I believe it is in the UK. The Pa 28 at 55 % power burns 6.0 Gals per hour therefore 4 gals = 40 minutes of fuel. You should not be in this situation by choice but hands up for all those perfect pilots that never once got tight on fuel due to unexpected headwinds or overly optimistic flight planning.
I stand by my assertion that in this situation it would be better to have 4 gals all in one tank than 2 gals in each wing tank......and you had better be at the stage of the flight where you are looking at your destination runway.