It is possible through the graphic settings ingame.
Sorry, that I have my German Battlefield 4 now enabled, but I'll translate the important words for you.
Ingame you may press ESCAPE to go into the menu. There you go to "Settings/Options/Preferences" and then onto the graphics tab. You'll see this extract:
Two white highlighted areas:
- In the tab menu on the top: I won't translate this for you, this is homework.
- "VOLLBILD-MODUS": Translates to "fullscreen mode". There we go!
In that second highlighted menu, you'll have three options for now:
- Fullscreen: Guess what!
- Windowed: Window mode, as you may wish it. You'll have the game in a seperate window.
- Borderless window: I guess you'll love it. The screen is a seperate window, but it has it's borders cut off. So it fits over your screen, just like fullscreen does, but it is in window state so that you can ALT+TAB out quickly, for example.
HINT: If you want to play with these settings, I recommend you to go to the TEST RANGE. There you can test out every vehicle and weapon - and settings when not online on competitive servers.
NOTE: Remind, that if you change settings in the Test Range, your computer may have a slightly better performance as there are 63 soldiers less in the field as usual. :)