I watched a video describing the practical differences in detail, and now...I still don't get it. They have the same max. theoretical speed. I got the impression Thunderbolt 4 has advantages over TB3 for docks and hubs, but for external SSDs TB3 is the way to go.
Roughly speaking, I think the progression is:
TB3: Carries multiple PCIe and DisplayPort signals over a single link. "Daisy chain" architecture - hubs/docks can only have one TB3 "out" port - but can have multiple internal PCIe devices - NVME SSDs, PCIe-to-USB3/Ethernet/SATA/whatever. Intel opened the protocol to the USB-IF who produced...
USB4: Basically TB3 by another name, but with added support for carrying multiple USB3.2 signals alongside the PCIe and DP & support for hubs with multiple downstream USB4 connections - basically making it more USB-like. However, lots of features - including some Thunderbolt 1/2/3 backwards compatibility and multi-display support - became optional extras.
TB4: No longer its own thing - "Thunderbolt" becomes an Intel certification and branding scheme for USB4. Many of the "optional" USB4 features are required for TB4, but it is still basically TB3 plus USB4's new USB-centric features for hubs and USB-over-Thunderbolt. Incidentally - as far as I can tell. the
only reason the"TB3/USB4" ports on M1/M2/M3 MacBooks aren't branded as TB4 is that they don't meet the "at least 2 displays via Thunderbolt" part of the TB4 spec.
USB4v2: Adds new 80Gbps speeds and PCIe V4 support to USB4.
TB5: As with TB4 - an Intel branded/certified version of USB4v2 with stricter minimum specs - which
is potentially faster than TB3/4.
The TB3 vs. TB4 debate boils down to the fact that TB4 doesn't offer any speed advantages for PCIe-based peripherals (like NVME SSDs, mechanical HDs or PCIe expansion boxes) andUSB4/TB4 peripheral chips (at least the first wave) were aimed at the new-style USB4/TB4 hubs, which were all about offering multiple downstream USB4/TB4 ports and "tunnelling" USB 3.x from the host's USB controllers (at the cost of using up PCIe lanes). So there was certainly a point where TB3 was actually
better for PCIe-based peripherals like NVME drives which could use all 4 potentially available PCIe lanes. If you have a call for an external SSD
plus multiple downstream TB4 ports then maybe... but if you're losing sleep over how much bandwidth your SSD is getting then you
don't want it sharing a TB controller with a hub offering 3 downstream TB ports...
TB5 offers PCIe4 support and (consequently) faster speeds for everything - but check that whatever SSD drive is inside can actually use that bandwidth...