[Ron Ramirez]

If you installed a 5Y3 and the radio works, then it was converted to use a 5Y3 by rewiring the rectifier socket.

5Y3 and 5Y4 are electrically identical, but the base connections of each pin are not identical. In other words, a 5Y3 is not a Plug and Play substitute for a 5Y4 and vice versa - the connection to each of the pins differs between the two.

So if it already working with a 5Y3, might as well leave it that way unless you are a stickler for authenticity.