You won't notice it unless you need a ton of layers at a very high resolution in something like Procreate, Affinity Photo/Designer, or Photoshop.
Safari tabs will eventually reload earlier with 4GB of RAM, but in practice I have never noticed it, and when I do go back far enough to an old tab, I usually prefer that it reloads so that I can access updated information because it was usually a day or two ago that I last looked at it. By comparison, on older iPhones back in the day you would be filling out a form in one tab, switch to another to check something, and going back to the same form it would reload and you'd lose everything you had written. This happened to me a ton when commenting on MacRumors and fact checking what I was saying or checking on a detail I wasn't sure about in another tab. Just switching between two tabs was a problem back then. It got better over time and isn't really an issue now. You're more likely to run into an issue with apps reloading. I think with 4GB of RAM it's something ridiculous, averaging around 20-30 before they start to reload, and the nice thing about a modern iPad is that most apps launch in a flash so you don't even really notice it when it happens. And since iOS auto saves things you don't lose anything if it does it.
It's also important to note that several years ago Apple added memory compression to iOS. I think it averages around 50%, so 4GB of RAM is actually around 6GB of RAM compared to iOS devices of the past. But Windows offers something similar nowadays so you can't really compare it like that, though I'm not certain exactly how efficient Windows is at it. On older devices memory compression could slow down the device a little, almost like paging to disk back in the day, but less noticeable. On a modern device you wouldn't ever notice it happening.
So again, in practice I don't think most people will notice it unless they're planning on keeping this iPad for 5-6 years. Feature creep over time tends to eat more RAM. Though if Apple dramatically overhauls the iPad for productivity in iOS 13, as is rumored, who knows what could happen. I think most people who are in the market for an iPad Pro device are the type who upgrade more frequently than that anyway. I plan on keeping my 12.9" 3rd gen iPad Pro for three years, which should be about two generations if you look at recent history. By then the iPad Pro will have 6-8GB of RAM, be faster than most Intel Macs, have a GPU as powerful as a PS4 Pro, start at 256GB up to 2TB, use OLED/mLED display tech, have an @3X super retina resolution, possibly even slimmer bezels, maybe even a second USB port—and probably also run some sort of lightweight version of macOS when docked.
Safari tabs will eventually reload earlier with 4GB of RAM, but in practice I have never noticed it, and when I do go back far enough to an old tab, I usually prefer that it reloads so that I can access updated information because it was usually a day or two ago that I last looked at it. By comparison, on older iPhones back in the day you would be filling out a form in one tab, switch to another to check something, and going back to the same form it would reload and you'd lose everything you had written. This happened to me a ton when commenting on MacRumors and fact checking what I was saying or checking on a detail I wasn't sure about in another tab. Just switching between two tabs was a problem back then. It got better over time and isn't really an issue now. You're more likely to run into an issue with apps reloading. I think with 4GB of RAM it's something ridiculous, averaging around 20-30 before they start to reload, and the nice thing about a modern iPad is that most apps launch in a flash so you don't even really notice it when it happens. And since iOS auto saves things you don't lose anything if it does it.
It's also important to note that several years ago Apple added memory compression to iOS. I think it averages around 50%, so 4GB of RAM is actually around 6GB of RAM compared to iOS devices of the past. But Windows offers something similar nowadays so you can't really compare it like that, though I'm not certain exactly how efficient Windows is at it. On older devices memory compression could slow down the device a little, almost like paging to disk back in the day, but less noticeable. On a modern device you wouldn't ever notice it happening.
So again, in practice I don't think most people will notice it unless they're planning on keeping this iPad for 5-6 years. Feature creep over time tends to eat more RAM. Though if Apple dramatically overhauls the iPad for productivity in iOS 13, as is rumored, who knows what could happen. I think most people who are in the market for an iPad Pro device are the type who upgrade more frequently than that anyway. I plan on keeping my 12.9" 3rd gen iPad Pro for three years, which should be about two generations if you look at recent history. By then the iPad Pro will have 6-8GB of RAM, be faster than most Intel Macs, have a GPU as powerful as a PS4 Pro, start at 256GB up to 2TB, use OLED/mLED display tech, have an @3X super retina resolution, possibly even slimmer bezels, maybe even a second USB port—and probably also run some sort of lightweight version of macOS when docked.
