Looks like a valid URL to me. The 4555 must be looking for some other kind of security that was not necessary on the 4345.
The next expiriment would be to open up the share to everyone on the network. Try to make it as publicly accessible as possible. There should be an "Everyone" group under the shares Security tab, make sure it has full access to the folder.
If that is still not successful then there could be some kind of new protocol introduced on the 4555 that was not used on teh 4345. The new protocol may have higher or slightly different security requirements when scanning to a folder. We'd have to pull out a spec sheet to compare and see if anything else can be disabled or tweaked on the printer to enable the scan to folder feature in your enviornment.