Are you saying it's got some extra valve that's separate from the main loader control valve?
Most loader control valves simply have an extra position on one of the the spools for the float function. Looking at the parts diagrams, the 4500 is like most. There were 2 different control valve assemblies available for the 4500, and both of them had a float position on one of the spools. One was the regular control valve with 2 spools and handles and the other was the a self-leveling version that had a third spool for the self-leveling feature, but the float feature was still just a position on one of the main handles.
If you're talking about a valve that's separate, with what looks like a push/pull arrangement with a knob, that sounds more like a diverter valve that would divert the hydraulics for some other function. Post some pictures of what you've got, and track down all of the lines and tell us where they go and we can probably help you figure out what it's for.
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