iPad Pros have been 4 GB for a long time now, but that memory hasn't often been fully utilized for most people. However, iPadOS 13 is game changer. 4 GB will be enough for most regular usage, but if you are into multimedia creation, I suspect that 6 GB will come in handy sooner rather than later. I think it says something that CURRENT high end iPad Pros are already 6 GB. Applications are already getting bigger. For example, LumaFusion 2.0 can now use 6 4K video tracks, with 18 audio tracks. Full-fledged Photoshop is also coming.
It would depend on whether or not you need these bigger apps, with multitasking. I don't often, but nonetheless I'm hoping the next generation will start at 6 GB. I think this is feasible since iPad Pros have been 4 GB for 4 years now, and iPadOS 13 and iOS 13 are out soon, with bare minimum memory requirements of 2 GB RAM for legacy iDevices. Furthermore, the new baseline for the consumer oriented 2018 non-Pro iPhone XR and 2019 non-Pro iPads is now 3 GB.
I currently have a 4 GB 10.5" Pro, but my wife wants it. Thus, I will upgrade to an 11", but will wait until the next iteration, hopefully with 6 GB RAM. However, I suspect it won't be released until spring 2020.