If you were making a list of reasons to get the phone and adding a weight to each item I would put 4G on the middle of your list. WiMax (the technology) will have a difficult time piercing walls making your signal spotty and unreliable. It will mature over a long period of time but possibly could be replaced by LTE later.
However,
as I was pointing out in another thread, remember speed tests are only a small piece of the puzzle when measuring your 'experience'. Latency is another factor. And, WiMax has a bigger pipe to push data through even if your overall speed isn't what you were hoping. Speed can also be affected by how much free time the CPU of your phone has at that moment. IE, it takes CPU to process those packets. Your phone may be in the middle of hopping towers or negotiating a new signal or (x,y,z). These things would have a minimal impact but you get the point.
If you are purchasing the Evo simply to see 15mb on a speed test using 4G you will be disappointed. But I would encourage anyone to not dismiss Sprints 3G data network. I've been a long time hater of Sprint overall (customer service, spotty coverage). I haven't used them in over a decade and always suggested people choose them last for service. However, their data network is probably one of the best out there. And, over the past three years they've done a good deal of work improving their spotty coverage.
Anyway... my point... The only _real_ way to test the phone is measure the experience overall. How responsive do web pages feel? How responsive do sites like Google and MSN load (where you know the other end isn't slow)? You have to measure these things yourself as its a personal decision and choice whether they feel snappy to you.
To me... 3G or 4G doesn't matter as I've been wildly surprised at how responsive the net has been compared to the incredible I was evaluating and my iPhone 3GS of the past two years. Kudos to Sprint in my humble opinion. *Golf Clap*