Ipad Pro 11 to replace a Mac Air??

You can take this or leave it, but much as I love love love my iPad Pro 11 2018 with 256 GB and Apple Magic Keyboard, and as much as Apple has moved to make iPad Pro more laptop like, it is still not a true laptop replacement. Yes, you can run a lot of applications on an iPad Pro, but they are still downgraded, not full featured applications like their laptop/computer version. You cannot download software to an iPad pro. If there is no app in the app store for what you need, you can't download it to an iPad.

You really need to determine what you need to be able to do on iPad Pro and see if the scaled down version of the programs you'll routinely use will be enough for your needs. I'll always have an iPad. For the forseseeable future, I'll also have a laptop.

It depends on what you need it for, it's a personal decision and nobody can tell you how much storage to by.

4G Mac air? What even is that?

What’s 4m memory?

that makes no sense and isn’t a specification for a MacBook

do you mean Storage or Memory? Both are in GB, not in M,

If you can have only one device, that is honestly the best solution. And you're most welcome.

4 megabytes isn't a term for memory...... maybe do research before buying a product