They aren't cross compatible if thats what you are asking. HPI didn't simply 'clone' the T4, they made their own model but clearly whoever designed it had a T4 sat on the desk at the time!
Anyway the best indicator of the difference if philosophy between the two trucks is to look at which bits are metal. The HPI has an all metal drivetrain, the T4 has a metal layshaft gear but the idler and diff case are plastic. Metal gears are stronger, which is good for hard landings under power - both trucks feature a slipper clutch that should protect the gears but only if set correctly. HPI expects their users to be too busy having fun and doing wheelies to tinker with slipper settings. In contrast the plastic T4 gearbox has a much lower rotating mass, so it will accelerate better which is useful on a race track. Racers fiddle with slipper settings anyway so slightly weaker gears isn't a problem there.
The shocks are the opposite - plastic bodied on the HPI, alloy on the T4. The T4's shocks are much more precise, smoother and more adjustable. However a bad crash will at worse rip the caps off the HPI shocks, whereas the T4 you risk bending a shock shaft.
The HPI comes with a slightly faster motor out of the box (4000kv vs 3300kv) and includes much higher gearing for top speed runs, whereas the T4 is geared quite low as stock for acceleration. There isn't much between the two motors though and with some gearing adjustment they can perform the same.
The result is that the T4 is a good truck if want finesse and you try to take care of stuff. Its not weak by any means and will survive the odd bad landing just fine but not outright abuse. The Firestorm meanwhile is ideal if you want to sling it over jumps without worrying where or how it lands, hack about having a laugh until the battery runs flat and then put it away 'til next time. Very different approaches.