Just low-man a Raid/Arena Ops/Wellspring/Dares and call it a day. Most Raid encounters can be completed with 4. Might take a touch of practice for a Raid, but it's way easier than you think. It just forces more folks on mechanics, which is never a bad thing. Whenever our group exceeds 3, we tend to stick to those activities. I get we don't have any direct 4 man PVE, but all of those offensives are good options for a challenge. I'd love to see Salvage, Menagarie, Sundial and Escalation Protocol make a comeback and hit the Arena Ops. It'd give us more variety for those activities. Additionally, they really ought to tie the OG Onslaught weapons to Arena Ops. Black Armory 3.0 is now tied to it, but the 2.0 isn't. You have to go play Onslaught, which I get, but isn't the most sensible. It'd be far better to deepen the Arena Ops loot-pool with both sets of Black Armory weapons, then give Onslaught an entirely unique loot pool. I know it's a bit of a pipe dream regarding loot, but I do firmly believe we'll see the Arena Ops playlist expand in the future. It's a great place to wind down after grinding more intense activities.

Unfortunately that wouldn’t make sense

Canon wise this doesn't work as fireteams are specifically said to be three or six people (I believe) Gameplay wise there'd be a bit of a balance issue, and I don't think increasing everyone's health by 33% would help a ton. A fourth person could make a lot of dungeons significantly easier, considering that they were designed to be completed by three or less people. An extra guy would really just be as clear.

You have 3 friends that still play this game? Mine left years ago 😂