The TR-4 is, in many ways, the end of the series of cars that started
with the TR-2; the -4A is, in many ways, the beginning of the series of
cars that led to the incredibly long-running TR-6.
The 4 and 4A share an engine: the trusty 2138 cc high-compression four
cylinder that was first deployed in the late 40s (I think) with smaller
cylinders. A lot of the body panels are the same, and a lot are
different. The frame and suspension are mostly different.
The most visible difference is that the 4A has independent rear
suspension (well, most of them do), where the 4 has a live axle. This
required a new frame, and that frame and suspension went on to do
service in the TR-250, -5 and early -6 (most suspension pieces are
interchangable; I know that at least one of the rear axles on my 4A
came from a 6, and the left front vertical link and lower A arm are from a 6).
The -250 took a 4A and dropped in the six cylinder engine. The -6 then
replaced the body shell, but kept frame, suspension and drive line.
More competitive? The 4A engine produced 104 bhp @ 4700 instead of 100
@4600, and 132 lb-ft @ 3000 instead of 127 @ 3350. It weighed 2240 lb
instead of 2128 lb, and came stock with wider tires, but I'm not sure
if that means wider wheels (my guess would be that the 4 had 4" wide
wheels, where the 4A has 4.5" wheels stock, which only really matters
if you're going to run in ES).
mjb believes that a 4A can be a national competitor in DSP, and he's
going to try to prove it. I'm sure he'll have something to say about 4
vs 4A, since he's run both.
My guess is that the IRS doesn't buy a lot for autocross, and unless
you run dead stock, you can do about the same engine development for either car.
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