C4, C9 and C10 are basically the same.
The C6 is a much heavier duty box found behind bigblock engines.
A C4 is fine for any standard to mild v8 Falcon.
Many racers use C4 as they sap less power than a C6, however it is a trade off for reliability.
C6 is a heavy duty trans, expensive to buy and repair but very strong
C4 & C10 are basically the same, the C4 was in cars up to XY and C10 was in XA onwards, C10 can be built to handle quite a lot of power and is a good transmission, C4 is not far behind.
If you are going to use this in an early falcon such as XP or earlier you can bolt a C4 bell housing onto a C10 which will give you more clearance from the body.
The C4 uses a smaller flywheel, torque convertor and bell housing than C10, however, if the engine has a lot of compression you may have trouble starting it as the flywheel is smaller
hope this helps