If you want to do something more serious, 4GB won’t last even a year. This includes video editing and music editing. 6GB and more is required, just like a desktop computer.
Be sure to state that as opinion
(same as what I'm saying here) because I have 10 or so programs running in the background all the time while editing RAW photos or large drawing/illustration files (often have 10-20+ layers.) I usually have Pixelmator and Procreate open while also listening to music, having safari, youtube, and facebook open and used cause I have ADD and sometimes take breaks between projects to refresh my brain.
I have also been editing 4K movies on the iPad with no issues or any slow down since the first gen. iPad Pro.
IMO 4GB is plenty of RAM, now sure, IF developers make their apps or programs bigger and with more and more features, or you have projects that are very large then it will not be as good as editing on an iMac with 16GB RAM. Thing is grabbing the one TB 6GB RAM option gives you a little more than 1GB of RAM to work with, (more is better, but the difference will be minimal.) 8GB-12-16 RAM is where everyone will notice a bigger difference.
I work with my own Photography company, where editing RAW photos is constant, I also do design and creative consultation for WB and I use my iPad (very often I use my Xs Max too and have no issue with the RAM AT ALL.) iOS is so efficient that these current 4GB iPads are about equal to a MacBook or iMac with 8GB of RAM. Obviously more RAM is always preferred but the iPads and iPhones are unique creatures due to their hardware design and great OS. the 2nd gen and 3rd gen iPP will be powerhouses for a good 7 or so more years depending on what you do. only last year did my iPad 1 get retired due to the processor and memory being so small.
*Again my opinion, but coming from a person who hasn't used a desktop computer to do work with for 7 years (Only using iPads and iPhones) the 4GB is still solid, but especially iPads with "A" processors that are A10 and up.
OG poster, I suggest you buy any of the current iPads (Best Buy has a great sale going) or buy the 2017 A10X iPads and you will have amazingly fast, efficient, and great tech for many years to come. iOS 12 helped boost that even further since Apple made iOS 12's purpose to have better performance and last longer on older and newer devices. iPad Air 2 uses the software and it is still capable of doing most anything I throw at it. My iPhone SE can do whatever I throw at it too.
It also depends on what work you do and how much you are going to throw at your device, but I wouldn't worry about current gen or 2-3 year old iPads to have any issue.