I have plonked along at 7kph often enough, which is 4.5mph, entirely doable, and personally I will take pedalling over walking anytime, its easier I find, not banging shins etc.
I've found that when I got down to the lowest gear (a little over 4 mph), especially on steep forest roads, I'd rather just walk the bike. Walking is a welcome change after hours of pedaling, using different muscles, etc.