The cockpit area should be pretty much the same. You'd need to get a tonneau with NO fasteners installed, of course, since the TR4 uses lots of Dzus fasteners at the rear, while later cars all used snaps.
Ideally, you might try to find someone with a TR6 tonneau in your area and lay it on your car to check fit. My only concern would be at the rear, not only due to the Dzus fasteners but the capping under them, but I still think it should work.