I think Lamplight is a great place for a couple to have a great cocktail and apps or a meal. We have teenagers now, and they love it too. The setting is amazing, just a cool vibe, and a lot of Pixar memorabilia to look at.
When our kids were young like yours is now, we would not have taken them there because it is not a fast place on service, and the vibe is not great for little kids. There is not much, if anything really, for young kids to do but sit quietly at the table and eat. They could look out at Incredicoaster and the Wheel from afar, but that wouldn't entertain most 4 year olds for very long. I would feel uncomfortable as a parent with a young child there, as it is not a place that a little kid running around while the adults eat would be typical. I would definitely recommend a character meal with your kiddo, and then an adult's visit to Lamplight, if you have someone who can stay back with your child. We have done Storyteller's a few times when the girls were younger, and they loved it. The characters are very interactive, and the kids are entertained constantly. I am not a fan of buffets at all, but you do what you have to do for the kids! And it gives them a chance to try new foods, which at 4 years old is important. I have not done Goofy's, so I can't compare the two, but I would definitely do a character meal!
We absolutely love Blue Bayou, but again, we did not start going there until our girls were in Middle School. It is not a "kids running around" vibe at all. It is very quiet so you can hear the crickets chirp in the "bayou". It is dark in there as well. I would highly recommend going there, just not with a 4 year old.
As for doing your character meal on a park day or on OBB day, I would do OBB day if that party is the only park time you will be doing that day. A park day is usually long and busy, so doing a character meal either chews up the early part of the park day when ride waits are the lowest, or you do it later in the day and risk a tired, cranky kiddo. I think lunch on OBB day is great, if your kid will sleep in that morning (mine never would) because then you might make it until later that night for the full OBB. If you do breakfast on OBB day, your kid may fizzle out early evening, just when you want to see and do all of the OBB stuff!
) and once we had the all clear I took my boys to Storytellers to celebrate. They loved the characters! And they also enjoyed the food. I did too so bonus there. Too expensive? Yes, but it was perfect that day.
Most days I hope for the best. 
