Enabling four (or more) clients to stream "simultaneously" is absolutely possible, but almost certainly
not by just upgrading a single AP to 4x4. I'll explain:
First off, for that extra spatial stream to benefit simultaneous download ability, you'd need to have MU-MIMO properly implemented and working in the 86U and
all clients involved, a very low probability (for more on why that is, read
Why You Don't Need MU-MIMO). Even if that were possible, 4x4 would only allow up to one additional client to download at the same time, so only up to three clients total on that one 5ghz radio (versus up to two on the 86U). The easiest and most certain way to increase simultaneous wifi download (and
upload as well) is simply by adding more radios (ie. a tri-band router, or potentially better still, multiple discrete APs), plus proper broadcast/channel management.
Looking at your gear in particular, beyond simple interference or signal quality issues, if I had to guess, the root of your client behavior will come down to poor/buggy code running on the Asus, possibly a dying 5Ghz/2.4Ghz chip (as much as people think this happens in consumer routers, I'd say it's actually very rare), certain service(s) on the router choking/stuttering the throughput, potential
bufferbloat on the WAN and/or wireless interfaces (measurable with a
DSLReports speedtest - test with a wired
and wireless client separately), and/or potentially buggy/crappy wifi chips in any of your clients. Most likely, it probably 86U-related, and if you haven't already, I would factory-reset, load the latest, proven-stable
Merlin build and re-configure
from scratch, being extremely careful not to enable
any extraneous services/features. Also, depending on the type and speed of your internet, and/or if your bufferbloat scores were a B- or lower), you may want to enable fq_codel on the WAN interface (search the Asus sub-forums on how to do that).
Beyond that, a change to a tri-band router may be the easiest way to minimize your issues. I also tend to find that Qualcomm wifi -- offered on certain consumer all-in-ones (which are unfortunately
not most Asus models) and higher-end wired APs -- often provides a smoother simultaneous wifi experience in general, all other items held equal.
With all of that understood, there is a partial savings' grace here, which actually comes from how streaming data is downloaded by most modern apps: it's not truly continuous, but rather spaced-out into "chunks", with pauses in between (example: YouTube or Netflix streaming will display download "peaks" every few seconds, with valleys of almost no throughput in between). This helps to transmit enough data to clients, even in-serial, to allow what feels like "simultaneous" streaming to at least several devices at a time in most small networks. Provided you don't have a faulty device(s) in the mix, per what I suspected above.
Hopefully, some of that helps.
