My understanding between TB4 and TB3 is that they're essentially the same, it's just that the standard of TB4 essentially mandates that the device must do all that TB3 maybe could do. Minimum bandwidth is increased and I think I read something about power delivery minimums as well. This eGPU chassis I bought came with it's own TB4 cable, which is actually the first Thunderbolt cable I've seen that specifically says "4" on it.
I assume the reason they supplied this is because, given what it does, an eGPU chassis is going to need to support some pretty bandwidth for a GPU. In my case though, I'm actually using this chassis not for a balls to the wall kick ass Graphics card, but actually to allow me to attach an old and very humble i/o card from Blackmagic. It's currently working just fine for that purpose.
Thing is, the supplied TB4 cable is pretty short and the chassis along with the ATX power supply mounted on it makes for a pretty hefty desk-space consuming setup. I'd like to move the whole setup somewhere fairly far off from the laptop to save me some precious desk space. I looked up 2m thunderbolt 4 cables which I understand is the longest distance you can get for TB4 and still maintain bandwidth and while it's not too bad, the prices are high for a cable. It occurs to me though that since I'm barely using a fraction of the available bandwidth anyway, could I use other, cheaper, long cables. USB4 comes up a lot in my search for 2m TB4 cables for example. (although they are mostly from AliExpress so don't know how good an idea it is to buy from them). If the chassis has TB4 controllers in it, as does the laptop to which it's attached, can one just put a USB4 cable between them? Are they physically different?
For that matter, since my bandwidth needs are so tiny, could I just find cheaper, longer TB3 cables?
You may need an active TB3/4 cable. 30 cm is typically the longest passive cable you’ll see. Active cables are more expensive, though less so than they used to be, and can handle longer runs like 3 meters.