The big jump in quality will come from the color gamut expansion with 4K, not from the resolution. The problem is that while 4K seems to be pretty happy, it's the color standard which seems to be all over the place. HDR has several standards associated with it, and projector manufacturers have really struggled with delivering on upgrades in the past decade.
DLP is almost entirely stagnant, if not devolving their technology. That is, they want to go smaller and cheaper, while dropping in quality instead of delivering a mid-tier product that is exceptional. They've had the same 1080p chip for years and years without any improvements, and while they own the entry level market (for good reason) Epson has made incredible improvements in the past few years, and their newest entry level products give DLP a run for the money.
The continued headache of fitting 8 million pixels on a 1" chip is a nightmare for developers. Only LCoS has really delivered on that promise at anything close to an affordable price to this point. But, IMO, you can get a nice Sony HW45ES today, and enjoy it for a few years, then get a new 4K projector 3-4 years down the road and have more quality than what any 4K projectors delivers today for twice the price.
I'm pretty sure that I will hang onto my W1070 for another 3-4 years before 4K will make sense and have enough standardization to be the way to go at a reasonable price.