https://wccftech.com/all-m1-mac-models-support-thunderbolt-3-not-thunderbolt-4/

During Apple’s 2020 WWDC preview, there was concern that by dropping Intel CPUs, the company would drop Thunderbolt 3 support as well. Fortunately, that isn’t the case with the new M1 Mac models Apple announced today, but if there is some confusion regarding Thunderbolt 3 and Thunderbolt 4 support, let us get that out of the way first. As nearly all of you know, the new M1 MacBook Air, M1 MacBook Pro, and M1 Mac mini are each equipped with two USB-C ports, and no, these support the older Thunderbolt 3 standard.

When it comes to the overall throughput, the Thunderbolt 3 standard is no different than Thunderbolt 4. Both protocols cap out at 40Gb/s. This could be one reason why Apple stuck with the previous standard since it would be less costly to implement and offers the same bandwidth as its successor. However, there are some notable improvements when talking about Thunderbolt 4. Intel earlier shared some details regarding the new standard, and with it, a list of improvements.