With any RAM increase you'll get some better performance, in the case of the 6GB 1TB... from all I've heard over the past 9 months... It helps a little. It helps make it so your safari, paint apps, and any program that refreshes when you don't use it... doesn't refresh constantly. In terms of CPU and GPU performance it's actually slower than the 4GB model. Many people believe the RAM is for managing the enormous 1TB storage. Many tech experts even have said that, but realistically some will still be available to the whole iPad system. In essence you get about 1GB of RAM, so obviously one more GB will be nice, but working in computers for the past 15 years, more RAM doesn't necessarily mean it will be "better." I have an iMac with 4GB RAM and no issues with it UNLESS I leave it be for days then jump back in, because it's older RAM it takes about 15-30 seconds for it to warm up and do it's job. I do heavy photo, video and audio editing on both my iPad and iMac and the iPad works without a single slow-down and I have 20-30 plus tabs or layers or whatnot going in the background when surfing safari (new desktop version) facebook, iTunes, procreate, affinity photo, Pixelmator, and photos running and since I've owned this iPad, I have not had ONE single time where it was slowed down or had to refresh.
To give some perspective, I have been using my iPads as my main drivers/computers since 2013. This new iPad with iOS 12 and the new iPadOS13 make it at least 2-4x better and more efficient than iOS 11 and on older iPads.
I say if you are worried about having 6 v 4, wait for a sale and get the 1TB for around the same price as the 512 is regularly. But honestly I LOVE my iPad 256GB with 4GB RAM... and I know a bump to 6GB is not worth 500-600 extra, or even worth 200 extra. More RAM is always nice, but not necessary, especially when it comes to Apple; their OSs and hardware layout aren't like other manufacturers, and they work seamlessly.
So yeah, I think 512GB with 4GB RAM is plenty for a good 5-7 years or more, (my gen 1 iPad just became not useable for art or anything heavy-is in 2017. But I can still use it for web browsing, movies, music, and other basic stuff, (that's 7 years for a device with 256MB of RAM) so I think 4GB will be fantastic for a good very-long while.
I'll update to the new iPad (MAYBE) if it's 8GB and the A13 or A14 Processor with all the bells and whistles. As it stand though, the iPad Pro 3 is the iPad I've been dreaming about since the first iPad.
YMMV, do what you feel is best, but don't let your mind think the 1TB is better than the others. IT'S NOT, it is just as cool.