Do I pay the 4% DLD fee again when Oqood becomes a title deed?

No. For off-plan purchases the 4% DLD registration fee is paid once, at Oqood/SPA stage, so there's no second 4% at title-deed conversion. At conversion you pay only small fixed charges, roughly AED 250 title-deed issuance plus about AED 250 site-plan/map and minor admin and knowledge fees (verify current amounts).