I just updated to the latest Rhino for Mac-Version 5.3 (5D191) and since I expirience the same problem with all files and also if I create a new file:
Every time I open a new file, I can see only two viewports instead of all four. I then have to find the border (completely on the right or on the left) of the viewport and have to drag it to the middle with the mouse. Even when I save a document with the adjusted viewport setup, it opens next time with only two viewports visible instead of all four.
I work on a iMac with OS 10.8.5
Thats what it looks like every time I create a new document or open a file…
I can then drag the windows smaller so that the other viewports appear (this time it was from left side… other times i had to drag from right…):