Should you switch to Thunderbolt 4 cables?

If you’ve got a recent Mac, particularly an M1 model, you’ll probably be wondering whether new Thunderbolt 4 (TB4) cables are worth buying, now that they’re becoming available. I just got my first couple, CalDigit 0.8 m length, and have been looking at what you get for around £/$/€ 25, compared with £/$/€ 21 for the cheapest certified TB3 cable of the same length.

Before looking at performance, you need to know what TB4 is claimed to bring that TB3 doesn’t.

The maximum bandwidth when operating in Thunderbolt mode remains unchanged at 40 Gbit/s, but TB4 supports USB4 as well as USB 3.2, 3.1, 3.0 and 2.0. I’ve yet to come across a peripheral which can do USB4, but that’s apparently based on TB3 and should also deliver up to 40 Gbit/s.

For external displays, the differences also aren’t too clear. TB3 supports DisplayPort 1.2 but should be able to drive two external 4K displays at up to 60 Hz. Although TB4 supports DisplayPort 1.4 with a higher bandwidth, Apple currently states that M1 Macs only support a single external display using their TB4 ports. That could be Apple’s 6K Pro Display XDR, but I can’t ever see myself sat in front of one of those.

The other ‘major’ improvement claimed for TB4 is support for Thunderbolt Alternate Mode USB hubs, also known as the Multi-port Accessory Architecture. However, the only such TB4 hub which I’ve been able to locate is the CalDigit Thunderbolt 4 Element Hub, which is in such great demand that you can’t buy one outside the US, and even there the next batch isn’t due until April.

As you might expect, connecting an external TB3 SSD such as a Samsung X5 using a TB4 cable brings no performance advantage at all. Write performance is primarily limited by thermal throttling, which halves transfer rate from the 2 GB/s attained during brief write tests to 0.9-1.2 GB/s when longer tests are run. That’s essentially the same irrespective of whether using a TB3 or TB4 cable. Read performance isn’t affected by thermal throttling though, and attains 2.4 GB/s on both TB4 and TB3 cables.

For the moment at least, until TB4/USB4 hubs become more widely available, there seems no good reason to pay a premium for a Thunderbolt 4 cable. Even when you’ve got a hub, the only place that a TB4 cable will be of any benefit is running between the port on your M1 Mac and that hub. What is more important is ensuring that – whether TB3 or TB4 – your cable is properly Intel Thunderbolt certified. That doesn’t mean that it has to be one of Apple’s pretty white ones, though.