Downgrading the host to 4.1 is not that easy and you may need to backup/restore the VM's and may also have to do some configuration changes. Anyway, with purchasing any vSphere edition you need to also purchase at least 1 year of subscription, which entitles you to upgrade free of charge. This means, that even if you purchase vSphere 4 you can upgrade the license keys in the "My VMware" portal to version 5 and continue using the current hosts without the need to downgrade.
André