if it took four years for a message to cross the void from where you are to the recipient, you certainly wouldn't want to wait a full eight years to see they didn't send a receipt message and only then retransmit.
eight years is some awful latency.
you'd probably want to send each message at something like a fibonacci over the months. so, gaps of (1, 1, 2, 3, 5, 8, etc) would mean sending the message on months (1, 2, 4, 7, 12, 20, 33, etc) until you got a confirmation message that they had received it. they would similarly want to send confirmations in the same sort of pattern until they stopped receiving copies of that message.
spreading the resends out over time would ensure not all of your bandwidth was going to retransmissions. you'd want that higher number of initial transmissions in hopes that enough of the message makes it across the void that they would have started sending receipts reasonably close to the four years the initial message would take to get there.
if you had the equivalent of a galactic fido-net system, it could be decades and lifetimes between messages sent to distant stars and messages sent back.