According to a tweet by the official Splatoon 2 Japanese twitter, the upcoming version 4.3 update will be releasing on the 5th December:

本日開催された「第4回スプラトゥーン甲子園」近畿大会の会場で、次回の更新データVer.4.3.0の配信予定日が発表されたぞ。
12月5日(水)に配信予定だ。
つづけて、更新データに含まれる内容を紹介していくぞ。 pic.twitter.com/90pNV13sHI

— Splatoon（スプラトゥーン） (@SplatoonJP) November 24, 2018

The patch notes so far for the update are the following:

New Gear Abilities:

• Cold Blooded and Bomb Defense Up will be combined into a new ability. Any gears with the old abilities will automatically be changed over

• Main power up: Increases performance of your main weapon depending on the weapon you use with it

New weapons:

• Custom Explosher: Point Sensor and Baller

• Ballpoint Splatling Nouveau: Squid Beakon and Ink Storm

• Blobblober Deco: Sprinkler and Suction Bomb Rush

• Nauticlus 79: Suction Bomb and Inkjet

Toni Kensa Collection Part 4:

• Details TBC

Update: Nintendo UK have posted a video regarding the update:

We will keep you all informed once we have more information to share on Splatoon 2 so stay tuned!