Can we purposely use our medisave, and then topup MA later for tax relief? The savings from relief should be more than the 4% interest earned, yes?

Yes, I used it for my mums hospital bills. This year, I topped up my MA to BHS and enjoyed the tax relief up to $8,000.

Not sure how u define "purposely" use MA. There is proper limits and eligibility rules for using the MA.