You can't do anything about T-Mobile's change in policy for tethering, but there are a few fixes you can try to regain 4G service on the HTC Sensation 4G. One simple fix requires you to head to network settings and switching to WCDMA/GSM Auto if it isn't already selected. If it is already selected, or doesn't work, you can try disabling mobile networks for a few seconds and then turn them back on. And if that fails to connect your unit with the T-Mobile's 4G HSPA+ network, try toggling to turn on airplane mode and then turning it off.
Right now, there is no word on whether or not this bug will require a software fix from HTC.