Is it true that it is easier to obtain a PhD in Europe than the US on average?
Short answer: No. Long answer: Determining the minimum bar for which a PhD can be awarded is a bad exercise. If you are merely working towards the minimum you will not successfully complete a PhD.
One thing to note is that PhDs in the US tend to be open ended (to the extent you can continue to get funding) while in Europe they tend to have fixed durations. This means in Europe the bar comes whether you are ready or not while in the US you get to chose when you defend. Neither is easier, just different.
The duration between US and Europe are not usually the same, mostly because in the US, you only need to have a Bachelor degree to enrol for a PhD (and you can get a Master during the PhD), while in Europe, it's usually required to have a Master degree before enrolling for a PhD. Since it can take two years to get a Master, you also need about 5 years after your Bachelor to get your PhD.
I don't know if you always get your PhD after a number of years, no matter what, but the point in Europe is that a PhD is not necessarily an advantage on the market, with respect to a Master (at least, that's true in France), and is mostly required for working in Academia. In Academia, nobody cares that you have a PhD (because, well, almost everybody got one), so your publications, references, contacts, your CV in general will make a difference, not your degree. Hence, it might be possible that you can get the degree after a while, knowing that it's useless anyway if you haven't done any original research (although in many places, it's now required to have a given number of publications before being able to submit a PhD thesis). But I wouldn't claim in any way that it's a common practice, and I know several persons who have dropped (or been suggested to drop ...) their PhD program.
As for your main question, I don't know if one can tell whether it's easier to get a PhD in Europe than in the US, because it's hard to define what is "easier". If your question is whether you can come to Europe, enrol to any PhD, just wait 4 years and get your PhD, the answer is no.
Remember that Europe is a big place, comprised of lots of different countries. It doesn't make sense to talk about a European "PhD". The mechanism and typical PhD lengths between countries can be completely different.
The next few comments relate to the UK PhD system.
You don't get a PhD automatically after 4 years. I can introduce you to a few people who will testify to this!
In the UK, a PhD doesn't involve a "taught" aspect. I believe that in the US, the first two(?) years typically involve examinations.
In most Universities I'm familiar with, PhD students do not have a heavy teaching load. In my department, PhD students are allowed to take some tutorials/marking for additional money, but this is on top of there usual PhD working week. The number of hours a PhD student can work is closely monitored. In particular, it's incredibly rare for a PhD student to take undergraduate lectures.
I believe it is common in the US for PhD student to run a lecture course.