Agreed, their bounties will likely go beyond 1B if Luffy gets 4B, but if he's stuck at somewhere b/w 2B–3B, then I can see at least one of them ending up below 1B.
Looking at the number of reasons they needed to hand over 1.5B to Luffy after 500M—It's higher than Zoro and Sanji's current bounties—I find it unlikely that Zoro and Sanji will get bounties close to 1.5 so easily.
I mean, Luffy's given 1.5B for:
- Beating Cracker
- Beating Katakuri
- Being the ring-leader of BM's assassination plan.
- Getting the Fire Tank Pirates, the Sun Pirates, as well as Germa under his wing.
- For having SH Grand Fleet.
- There are a couple of other reasons that I can't recall at the top of my head rn.
No way beating King or Queen alone compares to all of this^ unless the WG actually thinks the Calamities are that dangerous.