Regarding OFDMA, one key point to remember about 802.11ax (or Wi-Fi 6) is it's not labeled as Extremely High Throughput (802.11ac was labeled VHT for Very High Throughput), 802.11ax is labeled as HE for High Efficiency. The majority of frames in 802.11 are 300 Bytes or less so the theory is all of these frames can be consolidated in smaller RU's and sent at the same time, allowing more time slices for that one client that wants to download a movie. The purpose of the client device matters just as much as the OFDMA.
If all the clients are downloading movies, then they won't request smaller spectrum chunks, they are going to request the full channel. If all of the devices are simply checking in and no demand is placed, then you won't see any increase in speed, and with only 4 clients, you won't see much improvement in anything since you can do that in 802.11ac.
For more information, there is a podcast in the US called Clear To Send that spent most of 2019 discussing Wi-Fi 6 and broke down a lot of stuff around it and I think they did a really good job. You can find that here https://www.cleartosend.net/802-11ax-series/