You don't multiply packet size by interval, you divide (or multiply by the inverse). And baud is measured in bits. But the numbers come out close anyway. 3678 / (2.5 * 3600) * 1500 * 8 = 4904.
I just checked wikipedia and there's indeed a protocol for them, but I've never seen one in the wild. In my day it was 300, 1200, 2400, 9600, 14400, 16800, 19200, 28800, 33600, 56k ...
So, back on topic, wouldn't it be more likely he was channel bonding two 2400 baud modems?