Surprised I've never seen this DIY approach mentioned anywhere or thought of it before 🤔 - usually people end up going for those mini PCs that have multiple network cards soldered to the mobo itself

Compared to an actual 10gig switch, the power consumption might be high (unless the network card drivers have been well optimised by the devs, offloading as much traffic handling as possible to the 10gig cards' own CPUs). In this case just make sure you have a powerful enough CPU to handle that traffic, as well as handle that 4gig traffic traveling between the network cards over PCIe.

Some gotchas to look out for though:

• PCIe lane wiring... are they going straight to the CPU, or are they going via the chipset (or slightly slower, a PCIe switch connected to the chipset). The mobo manual can advise on this, ideally you'd want something with as much PCIe lanes connected to the CPU directly to get the full speed.
• Power consumption... touched on this earlier but one to be aware of, esp if you live somewhere where electricity is expen$iv€
• Noise... you might need to buy a fan to cool down the network cards depending on your traffic, and how much the OS driver offloads to your 10gig cards
• A backup... if you need to do changes to your DIY switch, make sure you have some way of accessing the internet
• Bridging... there may be an ideal/recommended way to set up bridging for multiple interfaces on the same network card, to take advantage of hardware offloading, allowing you to get 10gig traffic between two devices even though the PCIe lane is just 4gbit
• Traffic filtering... again just ensure your CPU can handle it, particularly for HTTP traffic. I only do filtering on DNS traffic due to having a weak CPU, works well enough to catch some ad/tracking services that employ nasty tricks to evade blocklists.

I only have 1gig hardware so can't really provide comparisons :( however there's a youtube channel called ServeTheHome that started measuring power consumption of almost all the hardware they test - if they've done 10gig switches recently then that should give you some pointers at least

Edit: fix formatting 🫠