Today PUBG Corp announced that they have started beta testing of their new experimental 4×4 map. To get your invite simply visit mapbeta.playbattlegrounds.com and sign in with your Steam account to receiver your Invite code.
Keep in mind that the experimental server is a 4.5GB download, and you must already own a copy of the base game on the steam account you redeem the code with. The first beta is currently running from 2nd April 7PM until 5th April 4AM (PDT).
Don’t forget to provide feedback on the PUBG Forums