Hi pix530,
Yes, the Hemnes daybed is a unique concept. The only potential issue is having too much material when the products are stacked together, creating an unsupportive twin mattress combination. This type of product is most commonly used with thinner polyfoam or latex mattresses.
If you’re keeping the 4" polyfoam mattress, my main concern would be that it is substantial enough in firmness to provide a proper base for any material you might place on top of it. For a smaller child, a medium to medium-firm latex topper certainly could be a good choice. I do agree that 18" total would be a very tall product and I would worry about her getting in and out of the daybed.
Thickness is mostly a by-product of mattress design, so I couldn’t tell you the reasoning behind a doctor recommending a specific thickness, unless it was an overall height issue for a specific bed set. There is more about the effect of thickness in [url=https://forum.mattressunderground.com/t/memory-foam-toppers-do-any-good-ones-exist]post #14 here[/url.
Something like this could certainly work well for a growing child. Would you then purchase two of these to get the height matching for the Hemnes, as the 6" would be thicker than the 4" Ikea foam?
Phoenix