4 litres per minute would be more than enough if you could get it into your lungs.
TSO requirements for supplemental oxygen in litres per minute are roughly:
1.3 at 15000'
1.9 at 20000'
2.4 at 25000'
2.7 at 30000'
3.3 at 35000'
2.3 at 40000'
So a walk-round bottle with 4 lpm flow will give you enough O2. The trouble is getting the mask on in time and at high cabin alt getting the O2 into your lungs. Without an appropriate mask the oxygen will just be flowing into the cabin at 4 lpm.
I got the figures from a certifying document for a quick donning mask so they are pretty trustworthy :-)