Xiaomi teased an ultrathin book-style foldables ahead of its launch in China on Friday. CEO Lei Jun said Xiaomi's upcoming Mix Fold 4 is 9.47mm thick when folded -- just shy of the world's thinnest commercially sold book-style foldable phone. 

The current thinnest foldable on the market is Honor's Magic V3, which was announced in China last week and measures 9.2mm thick when closed (excluding the camera bump). The Magic V3 beat the previous world record of 9.9mm, which was held by its predecessor, the Magic V2.

"Inspired by sports cars, we've engineered Xiaomi Mix Fold 4 with a revolutionary carbon architecture design," Jun wrote in a post on X, formerly known as Twitter. "100% carbon fiber material delivers not just an incredibly thin and light design, but also unmatched strength."

The announcement of the Mix Fold 4 comes days after Samsung unveiled its next-gen Galaxy Z Fold 6 phone, which features a slimmed-down design compared to its predecessor, the Galaxy Z Fold 5. However, it's 12.1mm thick when folded and weighs 236 grams, which means it's lagging in terms of portability compared to the Mix Fold 4, Magic V3, and even last year's Magic V2.

Xiaomi also said in a post on Weibo that the Mix Fold 4 will have four cameras, including two telephoto lenses, one of which is a periscope-style lens capable of 5x optical zoom as well as two macro lenses. The Mix Fold 4 would also have wireless charging capabilities, supporting up to 50-watt charging, two-way satellite communication and an IPX8 rating for water resistance. The Beijing-based company also announced the launch of the so-called Mix Flip, Xiaomi's first-ever clamshell-style phone, but it hasn't shared further details apart from its launch date on Friday.

While foldable phones still make up a small fraction of the overall smartphone market, their share of the pie is growing, according to research firms. The prohibitive price tag of foldable phones as well as concerns over their durability have been among the top hurdles coming in the way of their mass adoption. Samsung has long been the market leader in foldable phone shipments, but reports from research firms Counterpoint Research and IDC, show Huawei overtaking the South Korean firm for the first time ever in the first quarter of 2024.