The MC68000 CPU used in the Plus and SE only has 24 address bits. Twenty-four bits yields 16M (about 16 million) addresses. So they devoted 1/4 of the address space to RAM.
They probably could have gotten it to 8 MB without too much trouble....
But, for the original design, look at what the bits do. Any address in which the first two bits are zero can be ignored by all the hardware except the RAM. Then those two upper bits can be used as code for which addresses are directed to which I/O hardware.
If they had spent 8 MB on RAM or one half of the address space, then there would only be one bit to use for simple address decoding. Instead of decoding the upper two bits into four combinations, the hardware would look at the single bit which says, "RAM or Not RAM" and then it would also have to look at the lower bits in the cases where the answer was "Not RAM".
So keeping the RAM to 4 MB may have achieved a significant savings in address decoding hardware. I'm not certain it did in reality, because I don't have the Mac Plus address map sitting in front of me, but it's published in "The Guide to the Macintosh Family Hardware".