I agree with you questioning the purchase since the Surface Pro 6 Quad Core CPU and longer battery life make it an attractive option when looking to buy a Surface Pro. I do have to say that the Surface Pro 4 did improve greatly on the Surface Pro 3 and it finally made the Surface Pro a threat to the iPad "computer replacement". You will save a lot of money buying it for $650 so it's a great start if you can't get a 6. Especially at i7 16GB ram.
I think it depends on what your needs are. I've had my SP4 i5 since it came out. I don't see anything about the 5 or 6 that would make me want to upgrade. The battery life has improved, but I've never found myself needing more than the 4 hours or so that I get. I only use my SP4 for travel and miscellaneous use. I have desktops at home and work.
Yeah it really depends on what you need. I use my fanless sp4 for displaying sheet music and some light office use. It's perfect for that even though its three years old and only the m5 version.
I have a desktop gaming PC at home so that covers all my other needs. If you need to do processing heavy stuff I would buy at least something with a quad core cpu and 16gb ram. And I'd also wait for a USB c model.