Welcome to the Community - If you are not planning to move the entire environment at this time vSphere 5 I would just add ESXi 4.1 hosts to make up the new cluster. If you do decide to upgrade and use ESXi 5.5 and upgrade vCenter I would also plan for upgrading the other hosts to 5.5 - until you do upgrade you will be able to run existing hosts with ESX 4.1 -
ESXi 5.5 hosts can utilize VMFS3 - but if you add a new VMFS 5 datastore the hosts running under 4.1 will not be able to access that new VMFS datastore