The first item to check would be the installation instructions for the unit itself. If it requires a certain circuit size/wiring then that must be followed.

I don't know which backup heat you have, but looking at the Trane literature I have, the 10 KW I have says it needs a 60A breaker and #4 wiring.

Conductor ampacities depend on the material (copper vs. aluminum) and the insulation (temperature rating). #4 ranges from 65A to 95A depending on the type of conductor.
Now #6 can go anywhere from 50 to 75, and 55 is a common value (for type NM copper).

I assume we are talking about a 240V supply here.
10KVA (this unit is almost certainly entirely resistive) is 41A.
9.6KVA @ 54 amps would indicate a 177V supply (not a voltage you typically see in any configuration in the US).

Who is "informing" and "telling" you things?

The Trane salesperson said according to Trane specs 10 kw is really 9.6 kw and requires 54 amps. I currently have a # 4 aluminum wire up in the attic where this heat strip and air handler is going. I don't know conductor type, but it is in an unconditioned attic.

Again, your Trane rep is spouting nonsense. First I don't see anything in the specs of the thing that says that 10KW is really 9.6KW. Further, even if I bought the 9.6KW number 9.6 KW at 240V is not 54 amps. As stated it would indicate a 177V supply to get that which you don't find in typical US use (choices are 120, 208, 240, ...).

The temperature rating isn't really dependent on whether the space is conditioned or not. It's a metric of the insulation properties (If your attic gets up to even 60 degrees C you're in trouble anyhow).

#4 Al is rated at 55, 65, or 75 depending on just what sort of wire it is. But he might be right on the 55.

Of course you also have to figure in the load from the airhandler.

Could it be that the air handler is served by a single circuit? Then you would have to add amps for the blower motor, low voltage transformer, and other controls in the air handler. My air handler has a total of 20Kw heating divided between the two circuits . Both circuits are #6 wire, one is a 50 amp breaker and one is a 60 amp breaker to serve the blower and other systems.

" In my opinion, this is by NEC definition a continuous load so the conductors and the overcurrent device should be rated at 50 amps."

If the heat strip in on for 3 hours or more then there is a major problem with your system. Yes, the unit may run, off and on, for hours or days and use the heat strips, but it cycles on and off. It is like the burner in your oven. To maintain temp it turns the burner on and off while the roast is cooking. Otherwise it would gust continue to get hotter and hotter.

Again, the TRANE doc for an equivalent sized heat strip (don't know what he has) specifically says #6 copper and 60A breaker.