Thank you for all the comments so far, still on the fence at the moment - will probably see what WWDC brings and then decide after that.....
It does not feel exactly the same to me upgrading from a 12.9 M1 iPad Pro. The M4 13 inch feels snappier than my previous M1 and playing games I can see things I didn't see with my previous M1. Such as shadows and white powder coming from a pitched ball in my baseball game.It all just depends on how much you value screen quality. I personally would do it because I love inky blacks and high contrast displays. But if you don’t really care that much, it’s a waste of time and money to upgrade. Everything you do on your iPad will feel exactly the same.
Congratulations on your purchase and I'm glad to hear you are happy with it. I too went from a M1 11" Pro to the M4 256GB cellular model at launch and have been very happy with it and never considered a return. I have it in the Apple folio and just purchased the Pro pencil for it.Well, I decided to go ahead and upgrade and have had the 11" pro for a week now and am very happy with it. I didn't bother waiting for WWDC and am glad I didn't as I am not really too fussed about the AI capabilities and will probably turn a lot of them off anway.
I am absolutely loving the OLED screen, for me its a noticeable upgrade over the LCD screens and just makes the whole experience of using it nicer for me: photos "pop" and I can definitely see a massive improvement in HDR content.
One thing that did really surprise me was actually being able to notice a meaningful difference in the weight of the machine with case: whilst not exactly heavy, the previous version with a case did feel a tad on the bulky side for me and I have no such qualms with the latest model. With the landscape camera I am also holding it less and less in portrait, particularly when using Safari. Previously the majority of my usage was in portrait mode but now it is in the minority. Another (minor) upside for me is the improved battery life.
All in all I am very happy I made the upgrade, it is a noticeably nicer device to use for me.