Hi ChannelGSW,
They would be the “expert” about what would be suitable for their mattress although I would have otherwise thought that a 4" gap would be a little more than I would be comfortable with if there was no foam layer under the springs in the mattress.
Some economical options that you could use to even out the support with slats with wider gaps or add ventilation under a solid platform surface are listed in post #10 here.
Phoenix