I have more than 10 years of experience with cheap RV foam mattresses. My first take is your 4" foam is too thin. On the other hand "foam" comes in all kinds of performance ability.
Cheap foam is usually lower density, contains fillers like clay, and is sensitive to body heat. Clay separates from the foam causing loss of firmness over time. Body heat softens the foam so you wake up with sore spots after 8 hours. Memory foam is a marketing strategy to sell absolute lowest quality.
My solution was to add 3" of higher density, higher quality, custom size foam from Verlo Mattress Factory over the top of the factory delivered foam.
The mattress in my new cheap TT is 5" and was fairly firm. Two years later it sages in the middle but still doesn't leave me sleeping on the board underneath by morning. I will soon have to do something about it.
How do you know you are getting high quality foam? I don't know. The sales hype is way over the top. Marketing departments are high quality and as such are experts at deception and promotion.
No one reliable tests them and reports and if they did, the marketing departments would respond and change all the labels and terminology.
I wish you good luck and happy trails ahead!