I'm not aware of a way to transfer settings from ESX 4.1 to ESXi 5.1 other than with upgrading the host rather than reinstalling it. However, as mentioned before, the vCenter Server settings - e.g. folders, clusters, resource pools, permissions, ... - won't get lost. All you need to reconfigure are the host settings, storage access (depending on the storage connection) plus maybe host based DRS rules. So unless you have a large environment - in which case Host Profiles might be an option - a reconfiguration shouldn't take too much time.
André