How far is your PS5 from the Router and is there much in the way (Like Electrical Items etc.) that could cause the Wireless signal to drop? Electronics can and will cause some signal loss, but given that you tested with an Independent Broadband Tester and still got the 4Mbps I think this could be Speed issues.
Try to see if you can use a cable to plug it in and test the speed via the Settings on the PS5, then repeat the test if your LAN Speed is higher you may need to improve the Wireless, which is a common thing I see on these forums :).
You could also try other devices in the same area (Laptop, Mobile Phone etc. and see if the speeds are similar)