Our first was a 4 night, and we hated it and hadn't planned to cruise ever again. Our kids were maybe 6 and 8, didn't want to go to kids club after trying it a couple of times, and they ended up being kind of cranky, arguing, just wanting to stay in the room, the weather on our sea day wasn't great for deck time (cool, overcast, rain - in Feb), and also not great on CC day, I had some motion sickness, and with the early dining we felt like we were still full from lunch when having to go to dinner. Our youngest was crying on debarkation day, and everyone thought it was cute that he didn't want to get off the ship. But really he was in meltdown mode from being woken up too early (after waking up at the crack of dawn every other morning on the ship). We were so relieved to be off the ship and drive away from the port.
A couple of years later we decided to try again with applying lessons learned. I think I forgot some of the unpleasant stuff and realized I had a lot more knowledge to make things go better. Plus the kids were that much older. We chose a 7 day Western Caribbean, did an excursion at each stop, enjoyed late dinner, much better weather (late April/early May), I took Dramamine each night to prevent motion issues, and we dropped the expectation that the kids would spend much time in the kids club. We also showed up to port later so that we would be able to go right to our room to drop our carry-ons and not have to fight crowds on the deck that day, as we had all been a little overwhelmed by that the first time around.
We've since been on a 5 night, and I really prefer the 7 nights as a minimum. It's just too much work & too stressful for me to get there, the boarding process, only to get off 3-4 days later. I won't go on Wish with their current 3 or 4 night itinerary. We are booked on Treasure this spring.
Anyway, I don't know if that helped any. I think the 4 night was good in the sense that it prepared us for how to do it better. But I would never plan a 4 night now. Personally, I would go for the 7 night, but just brainstorm about what to do in the event your child hates the kids club and weather is not nice for deck activities. I mean there are things to do, but 4.5 year old's have a limited amount of patience for standing in line to meet characters, and likely isn't going to want to do trivia. For us, we decided 2nd time around if our kids wanted to watch TV in the room and play video games a good part of day on a sea day, we weren't going to worry about it ... it's their vacation too. That took a lot of stress off of us to just go with the flow and not insist they try and do things they weren't interested in. Also plan fun things to do off the ship when you're on a port day. We all enjoyed our excursions and that helped me feel like it was okay to let them veg out more on the sea days.