Possible drawbacks
As for the "drawbacks" part, there are different things to consider:
- Windows cannot read ext4
- Mac/Linux/*nix should have no trouble with it
- Using MTP, Windows won't have any trouble either
- If your ROM supports ext4, there should be no local problem
- You might want to switch to a different ROM one day -- that ROM might not support ext4 (unlikely, but not impossible)
There are several situations to consider, as well:
- If you one day get trouble with your card which cannot be solved "locally" (e.g. the card no longer mounts locally, or cannot be mounted via MTP/UMS, or you need to do a recovery of accidentally deleted files), you might need to attach it to your PC using a card reader (ext4 ↔ Windows: Fail!)
- If you lend your device to somebody only having Windows, he might encounter problems
- If you one day sell your device, you have to keep this in mind, too
Though for the last two situations, the solution might be simply to remove/replace the card, or re-format it (in the last case).
How to do it
Still here? OK, so the drawbacks seem to be negligible. There are multiple ways of how to get your card formatted with any file system: