You CAN install the OTA...BUT
There is a way to install the 4.67.651.3 OTA without loosing root but you have to meet the following criteria:
*You are rooted but still using the original odexed stock HTC/Sprint Rom
*You rooted your phone before the previous update (OTA 4.54.651.1) with Unrevoked and have verified that you have S-OFF. (Revolutionary may also work, but is untested)
*You are using Amon RA recovery (Clockwork may work too, but is untested)
Here is how:
First, do a nandroid...always do a nandroid
Be sure that any apps that you froze or deleted have been restored
Download the OTA, but DO NOT INSTALL IT YET
Move the OTA from the /cache folder (where it downloads to) to the root directory on your SD card
Boot into recovery (Amon RA) and flash the OTA
Verify the OTA installed properly and check to see that Fastboot is disabled
ReInstall Amon RA recovery-the OTA will probably break it, so it needs to be reinstalled.
ReFlash your Superuser file (PC36IMG.zip) from Amon RA recovery
Note that the bootloader installed by the new OTA will show as locked (***LOCKED (00W)***), but that has not seemed to have any negative impact, your phone will still be rooted and you can still boot to your installed recovery. Any apps that require root access will operate just fine. Hopefully HTC will be adding the Evo 4G to the bootloader unlock list so you can unlock it in the near future.