Re: Is it possible to upgrade ESXi 4.1 to ESXi 5.5 or latest one ?

1) Please refer upgrade guides from following URL, you got to upgrade 4.1 host to 5.0 first, and then you do have a way to upgrade it to 6.0 if you wish, provided that Hardware requirements are met.

http://pubs.vmware.com/vsphere-50/topic/com.vmware.ICbase/PDF/vsphere-esxi-vcenter-server-50-upgrade-guide.pdf

http://pubs.vmware.com/vsphere-60/topic/com.vmware.ICbase/PDF/vsphere-esxi-vcenter-server-60-upgrade-guide.pdf

2) What's new docs will help you to see what new things you can use along with upgraded vSphere

http://kb.vmware.com/selfservice/microsites/search.do?language=en_US&cmd=displayKC&externalId=2058665

http://www.vmware.com/files/pdf/vsphere/VMW-WP-vSPHR-Whats-New-6-0-PLTFRM.pdf