Common Misconceptions about Thunderbolt and USB4

USB4 vs Thunderbolt

USB4 vs Thunderbolt 3

USB4 is based on Thunderbolt 3 after Intel opened the Thunderbolt 3 specification to the USB forum.
USB4 supports carrying the USB, PCIe, and DisplayPort protocols. Thunderbolt 3 also supports the PCIe and DisplayPort protocols, however it lacks the USB protocol. Thunderbolt 3 docking stations and hubs achieve support for the USB protocol by utilizing dedicated USB controller chips in the device connected over the PCIe protocol.
USB4 supports hub topologies, however Thunderbolt 3 only supports daisy chain topologies. This means that Thunderbolt 3 docking stations can only have 1 Thunderbolt 3 output port whereas USB4 docking stations can have multiple.
USB4 revision 1.0 supports up to 40 Gbps including encoding overhead, however Thunderbolt 3 supports up to 40 Gbps excluding encoding overhead. This means that Thunderbolt 3 can theoretically achieve 3.125% higher speed than USB4 revision 1.0 despite USB4 revision 1.0 being newer. In practice USB4 controller chips are newer and more refined, enabling USB4 to achieve higher real world performance.
Thunderbolt 3 mandates many features that are optional in the USB4 standard.
USB4 supports an optional Thunderbolt 3 compatibility mode to enable compatibility between USB4 hosts (computers) and Thunderbolt 3 devices.

USB4 vs Thunderbolt 4 & 5

Thunderbolt 4 and 5 are Intel's implementations of USB4 40 Gbps and 80 Gbps respectively.
Thunderbolt has several features mandated that are optional in the USB4 specifications.
USB4 devices are fully compatible with Thunderbolt 4 hosts (computers) and hubs.
Thunderbolt 4 devices can be compatible with USB4 hosts and hubs if the USB4 host and hub(s) supports all features that the Thunderbolt 4 device requires.

Common Misconceptions

Myth #1: Thunderbolt 4 has crippled PCIe capabilities compared to Thunderbolt 3, making it poorly suited for eGPUs or high performance storage.

The JHL8440 - Intel's main Thunderbolt 4 device controller - only exposes a single lane of PCIe from the controller. This is a limitation of the JHL8440 controller rather than Thunderbolt 4. The JHL8440 has this limitation because it is intended for docking stations which typically need a lot of DisplayPort and USB bandwidth but not PCIe.

Myth #2: Thunderbolt 4 is always better than USB4.

Thunderbolt 4 mandates a higher minimum standard than USB4, however everything that Thunderbolt 4 supports is possible with USB4 even though many features aren't mandated.

The Windows Hardware Compatibility Program mandates USB4 ports must support DisplayPort Alternate Mode, PCIe tunneling, and Thunderbolt 3 compatibility mode. This helps ensure that real world implementations of USB4 on Windows devices meet a high standard for USB4 features.

Some real world implementations of USB4 are actually better than even the best implementations of Thunderbolt 4. For example external SSDs and eGPU docking stations based on the ASMedia ASM2464PD and ASM2464PDX USB4 chips can achieve higher real world bandwidth than any devices with Thunderbolt 3 or Thunderbolt 4 controller chips.

Myth #3: Thunderbolt 4 supports two DisplayPort signals however USB4 only supports one.

Support for two DisplayPort signals is the main exclusion from some real world USB4 implementations.

A single DisplayPort signal can carry up to 4 different display streams, so even with only one DisplayPort signal they can still drive a multi-monitor setup.

The USB4 implementations in the Apple M1, M2, and M3 processors only support a single DisplayPort signal.

The USB4 implementations in the AMD Ryzen 6000, 7000, 8000, and AI 300 series mobile processors do technically support two DisplayPort signals, however there have been mixed reports on the reliability of the second DisplayPort signal.