*sigh*
This is always a tough question. If I had the 8350, and you are reasonably happy with it, then I would keep it for a year or two more. The used market is tough right now for quality 4K models. The new market is mediocre. The 4010 is probably about as close to a direct replacement to the 8350 on the market. It has that nice long zoom lens which the 8350 is using. So, throw distance isn't going to be an issue.
But, the 5050 is a solid step up from the 4010 and is the premiere model for home theater use.
The jump from 1080p to 4K isn't night and day. You have a quality projector. You will need to upgrade your cabling to the projector, the AV receiver, and all your sources so that you can provide 4K source material to the projector. That's something to keep in mind as well.
At the end of the day, I agree with
@Projectorenthusiast that JVC delivers the best image in a dedicated theater space. But, if you are set on upgrading now, I would call the 4010 a very parallel move to the 8350 and not a huge upgrade. More of a resolution bump with HDR support. The 5050 is an actual all around upgrade. The JVC RS440 (DLA X590) would compete with the 5050 pretty well. The JVC RS540 (X790) would be a solid model that could best the 5050. Either of these units may be had under $3,000. But, they are hard to find on the used market.
The 4K native models are just expensive and exclusively come from JVC and Sony.
DLP is decent, but I wouldn't take them at $2,000+ unless it is the LG HU810 which is a laser model and ideal for brighter rooms.