Good topper for RV mattress.  To choose 3in. or 4in. is just an increase in thickness not comfort.  Im 55 and the 4in. gives my bed more height, but I like to get up into a bed. It might be too thick for someone who is shorter than me.
I like the double foam because there isnt that deep sinking feeling as to where you cant toss and turn.  Regular queen sheets on a short queen, fit the mattress fine to compensate for the extra height. 
Its a little wonky out of the box but give it time, it fluffed up perfectly.  No defects on the one I received. Comfort level is different for everyone so I wont criticize that, but  this topper works for me so far, and is an improvement over the firm RV mattress.