T4's used in US 914's and 411/412 were fuel injected so electric pump. Early T2 Bus ('72 up to about '75?) were dual Solex carbs so manual fuel pump. All T4 has place for the spin on oild filter near the horizontal larger than T1 air filter. Oil filler and dip stick are different on T4 than on T1 but varied even on T4 applications. A flat OEM T4 can fit in a speedster and I suspect a Spyder. I think in flat form it is easier to work on than with upright cooling. Upright conversion generally means loosing the thermostat too. Heat exchanges - and the mounting to the engine is very different - being on the bottom vs at either end. Near all T4 have an aluminum cases (a very few are early ones were magnesium) -- all OEM T1 were magnesium (but aftermarket one are aluminum now). On t4 you can remove/replace pushrod tubes with heads on - on T1 you need to remove heads (or use adjustable tubes).
T4:
