20p/ewe/week. Thats decent money - you could pay more for grazing turnips, say - can be up to 60p....works out at about £10/ewe/yr.
I wouldn't pay rent on 4 acres, not worth it, thats 'I'll bung you some lamb and fix the odd fence' territory.
Edited to add: thats sheep, of course...ponies probably considerably more cos one is a leisure thing and one is a business thing. Also, ponies tend to poach the ground - so Id ffer to re-seed and take them off a few months a year.