well, if you are upgrading or from your current 4.1 version to the latest 5.5.. you should understand that upgrading is a multistage process in which procedures must be performed in a particular order. Also there are number of things that you need to consider like the new licences, Check your hardware compactblity with VMware HCL, there will be SSO with latest version which I guess wasnt a part of your existing 4.1 infra.
If you decide to upgrade.. you can take a look at the VMware KB: Update sequence for vSphere 5.5 and its compatible VMware products
and follow the vSphere uhttp://pubs.vmware.com/vsphere-55/topic/com.vmware.ICbase/PDF/vsphere-esxi-vcenter-server-551-upgrade-guide.pdfpgrade guide that explains the detailed process, as well as Upgrading Best Practicies for vCenter and ESXi 5.5
IMO.. the ideal process should be :
1) upgrade your vCenter to the latest version
2) Add a new additional Host with esxi 5.5 in your cluster
3) Migrate/vmotion your VM's from one of your existing esx server to this host
4) Get the evcauated esx host reinstalled with esxi 5.5 and add it back to your cluster.
Repeat step 3 & 4 till all the hosts gets upgraded to the latest version in your cluster.