Hi Iregeo,
Your platform bed with curved slats that are 4" apart wouldn’t be an ideal choice for any type of mattress but it would probably be “OK” for mattresses that use a polyfoam support core (although it may change the feel and performance of the mattress compared to using the mattress on a more typical support system that doesn’t have any flex).
I would also make sure to check with the manufacturer or retailer of the mattress you purchase to make sure that your support system meets their warranty criteria.
Some of the suggestions in post #2 here may also be helpful.
It may be worth considering online options since you won’t need to test the mattress in person if it’s for guests. Some of the better lower budget online options that may be worth considering are listed at the end of post #4 here and the posts it links to.
Phoenix