Get real, we've only been studying radioactive decay patterns for less than 100 years, the proposition that radioactive decay patterns have occurred at exactly the same rate for billions of years is nothing more than a theory, with no potential evidence to back it up.
We don't need to be around for very long to observe consistent patterns that appear to not have any variation without external influences. If you observe a ball on a flat plane for an hour, notice that in that time it rolled 2 kilometers without any variation in its speed, and you see before you the flat plane extending as far as you can see in both directions with little to no real variance, it takes simple algebra to predict
exactly where the ball was 4 billion years ago, and
exactly where it will be 4 billion years from now. While there is always room for some doubt, there's simply no information to suggest the ball would be anywhere except where you predicted.
While I'm not educated on the intricacies of using radioactive decay for study, I would guess that getting the ~4.6 billion years out of the century of study involves the same manner of deduction.