I would like to offer some reasoning and speculation as to why the Evo does not necessarily need Froyo, in order to support 4G.
The only factual information we know at this point, is what has been released in the SDK and documentation to developers. And all that information really is is a programmatic workaround for 2.1. Comments in the code snippet offered indicate that until Froyo is released, this is the workaround that will need to be used.
Now, it is being argued that Android 2.1 does not support WiMAX. In objected oriented development, you will usually see various "layers" developed independently... your app layer, business layer, data layer, etc. Each provide a different function and offer abstraction to other layers. For example, code that drives the UI doesn't necessarily care how database calls are made to get information to display. It simply makes a function call with a handful of parameters and gets back the dataset - the data layer takes care of the dirty work.
Along those lines, what is to prevent HTC/Android from implementing minor modifications to 2.1 to support the new WiMax radio? Given the SDK documentation, it is implied that the exist OS is fully capable of utilizing the hardware as long as the application in question is made aware of the new network type. The rest of it is hidden in other layers which HTC/Android would have modified for the purposes of the Evo working with 2.1.
While I too, would love to get Froyo with the Evo, I do not believe that the little solid information that has been publically revealed provides enough evidence to indicate as such. The SDK release is evidence that the 2.1 infrastructure can more than handle WiMax as simply another networkType.