Straight Talk, H2O should work including data using a APN changer, but both services will most likely require jailbreak and CommCenter patch to adjust APN settings for MMS, so install TetherMe.
The Sprint 4S will require to be on 5.0.1 to support Gevey Ultra S unlock to work with GSM networks in the US. I would wait for the 5.1 untethered jailbreak to attempt such since a mistake would force a update to 5.1, and since the baseband in 5.1 is not supported by Gevey, do not update yet(if you happen to be on 5.0.1 which Gevey Ultra S CDMA requires). There might be a solution to preserve the baseband when updating to 5.1 for untethered jailbreak in the near future and hopefully soon a SHSH blobs tool for downgrading of the A5 products like the 4S. If already on 5.1 you're out of luck until Gevey develops another hack for the 2.0.10 baseband.