Are you looking at staying on site? You can just switch your search on Disney's site to indicate you're booking for 5 people to see the options, even if you're only traveling as 4.
For the values, you're looking at suites at AoA or music. Unfortunately, you end up paying at about deluxe prices for the value suites.
Moderates you're looking at POR or CBR fifth sleepers. They have the little pull down murphy bed, which I would say is best up through preteen or a petite teenager. (You didn't say your kid's ages.)
Most of the deluxes and many studio villas have the 5 person layout, including Poly. Deluxes will have a chair or couch that sleeps your 5th person. Studio villas, it's a regular bed, a pull out sofa, and then also a Murphy bed similar to POR/CBR (murphy here subject to the same height/size concerns).
If you like Poly, you can certainly stay with that!