You have to purchase the transmitter
that matches the frequency bandsplit and companding scheme of the receiver
you acquired. I'm not fluent with the A-T line, but unless you know for a fact
the 1000 and 4000 series utilizes the same companding circuit, you'll have to
buy the 4000 transmitter in the appropriate bandsplit.
Additionally, frequency selection is
based on one's own geographic location and RF environment. The only
generalization to be offered is to buy the bandsplit that will cover at least
one, if not both of the two reserved channels for wireless mic use, which again,
is based your geographic location.
Henry Cohen