The length should be known in advance in order to be written, so the message cannot be incrementally written. You need more complex framing scheme like Consistent Overhead Byte Stuffing for that. And many applications do want a variable number of length bytes because i) 4 bytes is actually too long for short messages and ii) some message can exceed 2^32 bytes. Not to say the generic varint encoding is good for this purpose, though [1].

[1] If you ever have to design one, make sure that reading the first byte is enough to determine the number of subsequent length bytes.