I cannot recommend purchasing Things until v4 is released. The company has refused to make an official policy that would give free updates to anyone who purchases even a day before Things 4's potential announcment.
If you wait for version 4, there's a good chance the new suite of apps (Mac, iPhone, and iPad are separate purchases) will be discounted across the board for for everyone, regardless of whether you're an existing customer or not.
In 2017 the popular macOS to-do app Things 3 was released, coming 5 years after the release of Things 2, which was released 4 years after v1. It has now been 5 years since the release of v3, leaving many wondering if v4 is in the works.
To me and others online, it looks very likely that Things 4 is now in the works. When asked if they have a policy regarding updates for customers who purchase shortly before a major release, they gave the classic PR "we don't have anything to announce" line, but added that Things 3 has their "full support." I would be much more comfortable to buy now if it had their full attention, but they never acknowledged this when I brought it up. When you combine this with their December call for beta testers, lack of any "in progress" items on their official status board, and lack of blog posts for over 6 months, it's hard to believe they don't have something new in the works.
If Cultured Code could have a standing policy that guaranteed customers who buy in the weeks before a major update aren't forced to buy again, they'd be able to give customers an answer to their concerns without having to announce anything. I know the App Store doesn't make it easy for developers to provide paid updates, but for a company their size there's no excuse not to put in the effort to make sure their users aren't the ones burned. For v3 they made this effort, but it sounds like things have changed. In my conversation they initially claimed their only option for v3 was to discount, only admitting to the free updates they offered to recent buyers (claiming it was annoying to wait for a batch of free coupons) after I pointed out that it was also an option.
Things is a great app I hope to return to someday, but for now I'm happy to continue using the Reminders app as well as try out OmniFocus. Things is worth the price, but I wish they'd improve their upgrade policy because I'm not sure it's worth twice the price you might pay if you buy now and v4 is released tomorrow. Unless there's no alternative you can use in the meantime, I'd suggest waiting. It might be a long wait, but it doesn't seem logical to buy an app that gives strong signals an update is on the way.