Mario Kart 8 Deluxe Booster Course Wave 4 will be released on Thursday, March 9th, 2023.  

Check below for the full  N-E guess release... (It's like a press release, with added ambiguity)

Mario Kart 8 Deluxe – Booster Course Pass Wave 4 Approaches The Starting Line On 9th March.

The fourth wave of the Mario Kart 8 Deluxe – Booster Course Pass DLC adds eight more courses to Mario Kart 8 Deluxe on Nintendo Switch

2nd March, 2023 – Rev up your engines, because eight more courses are making their way to Mario Kart 8 Deluxe on 9th March with the Mario Kart 8 Deluxe – Booster Course Pass paid DLC for Nintendo Switch. Wave 4 features another off-piste snow-covered slalom in Wii DK Summit. Plus, you can sail merrilly by in Amsterdam Drift from Mario Kart Tour and explore Yoshi's Island like never before in a brand new course to name just a few. To see all Wave 4 courses in action, check out the Mario Kart 8 Deluxe – Booster Course Pass Wave 4 announcement trailer.

Wave 4 includes the Fruit Cup and Boomerang Cup, with fast and familiar courses appearing from the Mario Kart series across the Game Boy Advance, Nintendo DS, GameCube, and Wii installments, along with the Mario Kart Tour mobile game. Each course can be played locally or online. The Cups are divided as follows...

Fruit Cup

• Amsterdam Drift (Tour)
• Riverside Park (GBA)
• DK Summit (Wii)
• Yoshi’s Island

Boomerang Cup

• Bangkok Rush (Tour)
• Mario Circuit (DS)
• Waluigi Stadium (Double Dash)
• Singapore Speedway (Tour)

Plus, for the first time since Mario Kart: Double Dash!! a returning character will be joining the racing roster, in the form of Birdo, who originally appeared in Super Mario Bros. 2 on the Nintendo Entertainment System in 1989.

Wave 4 will join Wave 3, which is already available and features eight courses, including GBA Boo Lake, 3DS Rock Rock Mountain, Wii Maple Treeway, DS Peach Gardens and 3DS Rainbow Road, as well as London Loop, and Berlin Byways from Mario Kart Tour. The Mario Kart 8 Deluxe – Booster Course Pass includes six separate waves, with eight courses each, which will all be released by the end of 2023.

Supercharge Mario Kart 8 Deluxe with the Mario Kart 8 Deluxe – Booster Course Pass to experience more ramps, turns and jumps across a total of 48 remastered courses as each of the six waves becomes available. Players can enjoy all six waves of the Mario Kart 8 Deluxe – Booster Course Pass at no additional cost with a paid Nintendo Switch Online + Expansion Pack membership, or by purchasing the Mario Kart 8 Deluxe – Booster Course Pass available in Nintendo eShop as a separate purchase.

Mario Kart 8 Deluxe – Booster Course Pass Wave 4 arrives on 9th March, bringing eight more fast and frenetic courses to Mario Kart 8 Deluxe on Nintendo Switch. There will be a selection of familiar favourites from across the Mario Kart series. The Mario Kart 8 Deluxe – Booster Course Pass includes six waves, with eight courses each, which will all be released by the end of 2023.

And here's the original video...

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