I have used this program since 2002 and still use it all of the time. This is much better than any type of on screen ruler. With changeable colors the calipers will adapt to any background color so you can see them.
I use them to measure things on a website so it stays consistent.
This says free trial then you must pay. That is a yes and a no. You do not have to pay once the 30 days are over. There are a couple of features you get extra when you do pay but nothing that I have ever needed. (I did pay though because this is an awesome program) If you don't pay you can still use them but you must click a register later box each time you open the program. The only bad part is the register and no thanks button changes between two different positions each time you open the program.
As for Version 3 and Version 4 I would rather use Version 2. Nothing wrong with the other versions but I do not need the 360 degree rotation so using version 2 keeps that out of my face.
This program will lock out a lot of memory at first it is a small amount but the longer you use it the more resources it takes but it release the memory as soon as it is shut down. Just something to keep in mind if things seem to get a little sluggish. If it was not for this memory issue this would get 5 stars.