Can we actively ask an editor to take a decision?
You can, but there's no guarantee the editor will make a decision. It's her journal - until she's satisfied nothing gets published.

A few things to think about:


I personally find it pretty unusual to invite a fourth reviewer for a paper that already has three reviews. I might invite more reviewers if the reviews I received were poor or if I didn't have enough reviews to be confident the first time round, which doesn't seem like the case here. Hence I suspect the "fourth review" was written by the third reviewer.
It's true that this can go on and on, but it's more likely that you're already at least halfway to a final decision. My experience is that very few papers reach four rounds of revisions. 
If the paper-to-editor ratio is high, then it's possible the editor is operating on autopilot. If you ask the editor to make a decision quickly, your paper could come under greater scrutiny. This could e.g. lead to the editor inserting some comments of her own or challenging some of your revisions that the 1st/2nd reviewer has already approved of. Conversely if the paper-to-editor ratio is low, then your request is much less likely to change anything.
People don't like to be told what to do, especially in this situation where the editor is the person in charge of the journal. If you decide to ask for a decision, write something non-accusatory, e.g. "we've been through two rounds of major revisions. When can we expect a decision?"