4500 series don't support "stacking" although some of the later sups do support VSS, which does have a pair run as one logical device. If you're using HSRP, it's likely you're not using VSS.
If you are running VSS, there's a different procedure for updating their IOS. (As 4500s might have dual sups, there's also procedures for upgrading their IOS to avoid or minimize down time.)
I haven't checked Leo's link, but generally, there's a special command to upgrade all stack members IOS. Your 3650 stack might offer an optional rolling upgrade procedure, in concept, another method to minimize any downtime during the IOS upgrade.
