Latest version of ESXi is not compatible with my old Dell T7400. 4.1 installs and runs, but the license number I got for my 6.7 download won't work with my 4.1 install.
Is it possible to still get a license for 4.1?
VMware does not issue keys for versions of products which are past end of general support or availability, and 4.1 most certainly is both.
I would try 5.5, it may install and you should be able to downgrade your key to 5.5 so give it a shot.