It's been established previously that Tony has a photographic memory. However, here he writes down the helicopter's four-digit registration number instead of memorizing it.
When Tony contacts the police on the helicopter radio, the registration number he gives doesn't match the one that was shown earlier.
The registration number 093G is not a complete one. It should start with an "N". The actual number is N3093G and belonged to a 1965 Bell 47G-4 serial number 3360 until 12/23/1981.
There is a fifth robber: the pilot who flew the helicopter to pick up the other four. He can be seen in the background when the other robbers discover the money has been switched. However, the pilot is never seen or referred to again.