ontime
I don't get this at all and I feel quite unhappy about it.
Rest assured that you have not lost out on anything. Well, not that much anyway.
What you think of as X4 is now just called Sonar.
See here for details. It is the successor to X3 and you can upgrade to it for 50$, 100$ or 200$ depending on the versions involved in the upgrade.
See here for those details (scroll down the page to Pricing).
Unfortunately, Sonar is somewhat more expensive than X3 and you'll have to study up on it to determine what's best for you.
Don't be put off by the term "subscription" flying around. You can pay for your upgrade in a single payment just like you've always done or, which is new this time around, you can pay for it in 12 instalments for a fee. Once it's paid for, you can use the program for the rest of your life. The first year you get updates for free, after that you have to pay for them.
Sonar is not available yet, but rumor has it that it will be release on the 21st of this month.