No need to worry here. BlueMarble 4 has the same requirements as version 3, meaning on "high res" it is using the high texture quality of Civ4 when it was released - before Firaxis later lowered memory requirements via lower res images through patches. So who can use BM3 should be able to use BM4 with the same settings. Who has problems with BM3 or BM4 on "high res" should try the "low res" installation.
Uninstalling BM3 for BM4 is not necessary; BM4 adds one more file to the user area than BM3 did, so all BM3 files will be overwritten anyway. In the case where you do not like BM4 and want to revert back to BM3 you should uninstall BM4 first, so that all its files are uninstalled before you install BM3.