Around here, the professional dog sitters charge $12 - $16 per visit, per dog!!!!!
That would mean, with 4 dogs, and at least 4 potty breaks per day, I'd be paying about $240 PER DAY!!!!
With four dogs, I'd be happy to find someone to watch all of them, and stay at my house, for $100 bucks a weekend. But considering that you only have one dogs, and that this person is a friend, I'd think $50 to $75 would be plenty!