Well, that was your first mistake as X customer service seems very uninformed with the actual onboard experience. @DaKahuna nailed it and I'll expand...
1) Prices for devices are in your cruise planner, it does get slightly cheaper (per device) with the more devices and is not a fixed amount
2) If you buy the 4 device plan, you can use up to 4 devices at the same time
3) Your actual devices do not matter, you can bring 8 devices and log them all in at different times
BONUS, some devices (PC laptops, Pixels, etc) have a hotspot feature that will turn an incoming wifi signal into a hotspot allows you to connect multiple devices. Unfortunately the iPhone's hotspot doesn't work that way and requires cell phone reception