I used to have an Amazon Kindle Fire HD 8.9". Around this time last year I was debating replacing it with another Kindle or an Android tablet. I hadn't even considered an SP4 until I went to Best Buy and actually started playing around with one. That changed everything for me and I'm glad I made the decision I did. I use my SP4 a lot more than I use my desktop now, it has a lot more functionality than a standard tablet, and is more portable than a traditional laptop.
As for waiting or not, it's possible Microsoft could release a new model later this year. I'm sure they're working on something, but the question is when and I don't think you'll ever get a firm answer. The major benefit from waiting would be a significant price cut on the SP4 when a new model is released. From a hardware perspective though, the SP4 still has a lot of life left in it and any future model would probably just be a minor upgrade.
EDIT: Oh, and you can run Android apps on it if you're willing to use an emulator. I use one for a handful of apps that just aren't available on Windows or don't have an equivalent, but I try and stick to stuff that's in the Windows Store as much as possible.