Why length used to cost you speed
A cable this fast has to fight physics, and for years the price of a longer lead was slower data. Eaton's Thunderbolt 4 explainer spells out the old rule for Thunderbolt 3: "a passive Thunderbolt 3 cable can only transmit at the full 40 Gb/s data rate over a distance of 0.5 m (18 inches)," and "at 2 meters, a passive cable will limit data transmission to 20 Gb/s." So a bargain-bin 2m lead didn't fail outright; it quietly halved your bandwidth, which is exactly the kind of fault you never see until a fast drive crawls or a second screen refuses to wake. The only way to hold the full speed over a longer run was to pay more: "for the maximum 40 Gb/s throughput up to 2 meters, you'll need an active Thunderbolt 3 cable," as the same page puts it. Length, in other words, was a spec that mattered as much as the port.
What Thunderbolt 4 changed
The headline improvement in Thunderbolt 4 is that the length trap mostly went away. Intel, which owns the standard, states that "Thunderbolt 4 cables provide top transfer speeds at up to 2 meters," so a lead that carries the Thunderbolt 4 mark is guaranteed to hold the full 40Gbps out to that length rather than tailing off partway. Eaton frames it as the direct answer to the Thunderbolt 3 problem above: "with the introduction of Thunderbolt 4, passive cables will be able to transmit up to 2 m at 40 Gb/s." Up to 2 metres, you don't have to read a bandwidth-versus-length chart or work out whether a cable is passive or active; if it is certified for Thunderbolt 4 and no longer than 2m, it carries everything the port can send. That is why the certification is worth looking for on the packaging: it is the shorthand that used to take a spec sheet to confirm.
Passive, active, and when the difference bites
Passive versus active is a question you can ignore until the run goes past 2 metres. A passive cable is a plain set of wires; an active one has electronics built into the cable to keep the signal clean over distance. Plugable describes its own long lead as having "specialized Intel circuits, called retimers, along the data path that act as relay stations to boost the signal," and that boosting is what buys the extra length. There is a modern wrinkle worth knowing if you remember the Thunderbolt 3 era, when an active cable came with a catch. As Plugable puts it, "you can get an active Thunderbolt 3 cable, but it only transmits Thunderbolt data, not USB," so it would drive a Thunderbolt dock yet refuse an ordinary USB device. That trade-off is gone on the current generation: Plugable's advice is that "if you need an active cable, stick with Thunderbolt 4, it's more versatile," because an active Thunderbolt 4 cable still carries USB as well.
How long a cable do you actually need
For most desks this is a shorter question than it looks. A laptop sitting beside a dock or a monitor is well inside a metre, and a tidy run to a stand or a shelf is comfortably under two, so a 1m or 2m certified cable covers it, and there's nothing to agonise over. You only reach for an active cable when the gear is genuinely far apart: a dock on the far side of a room, a display wall, a lead threaded through a desk grommet and along a cable tray. Those situations are real, but they're the exception, and an active lead costs noticeably more for the same length, so it's worth confirming you actually need the distance before paying for it.
When you do come to buy, the Thunderbolt 4 cable guide sticks to leads that carry the certification this page keeps pointing at, so you are not left guessing at the full 40Gbps. If you can't yet say whether those ports are Thunderbolt or ordinary USB-C, settle that first with the USB4 vs Thunderbolt 4 explainer: the length rules here only govern a Thunderbolt 4 link, not every USB-C lead. When the whole reason for a long cable is a dock parked out of the way, the USB-C docking station guide covers the kind built to sit at the far end of a single lead.
FAQ
Q: How long can a Thunderbolt 4 cable be? A: Up to 2 metres at the full 40Gbps. Intel states that "Thunderbolt 4 cables provide top transfer speeds at up to 2 meters," and Eaton notes that Thunderbolt 4 passive cables "will be able to transmit up to 2 m at 40 Gb/s," so any certified lead within that length carries the port's full speed. To go further than 2m you need an active cable.
Q: Do I need an active Thunderbolt 4 cable? A: Only if you need more than 2 metres. Under 2m a certified cable already holds the full speed without going active, so an active one is money spent on distance you may not use. Active cables carry electronics Plugable calls "retimers" that boost the signal over a longer run, which is what you are paying the premium for.
Q: Is a longer Thunderbolt cable slower? A: It was on Thunderbolt 3, where Eaton says a passive cable dropped from 40 Gb/s at half a metre to 20 Gb/s at 2 metres. Thunderbolt 4 is what fixed that: a certified Thunderbolt 4 cable holds the full 40Gbps all the way to 2m, and only beyond 2m does an active cable become necessary.
Q: Will an old active Thunderbolt 3 cable work for everything? A: Not quite. Plugable points out that an active Thunderbolt 3 cable "only transmits Thunderbolt data, not USB," so it can leave ordinary USB devices dead. An active Thunderbolt 4 cable does not have that limit, which is why Plugable's advice for a long lead is to "stick with Thunderbolt 4."