[Old] Ra has said the human form is most common in 5th-density entities...

[anagogy]

Cool! And we get to be whole new beings in 4th density! I am so excited!

Well, sort of. Depends on what you evolve from. A 3rd density human race that evolved into a 4th density human race would most likely look very similar to the race of genetic heritage from whence it came.

But you can be a whole new being in 3rd density too. There are an infinity of alternate 3rd density vehicles, on other worlds, that one could incarnate into. Same goes for 4th density. You have probably been other life-forms in other lives. Most wanderers have.