It's 4mu when no stretch or shrink is applied/necessary. It can be shrunk down to 0mu (4mu minus 4mu) or stretch out to 6mu (4mu plus 2mu), depending on what is optimal according to TeX's paragraph algorithm.
Although I've read these two posts:
Texmaker vs TeXstudio (comparison)
I would like to ask again specifically about Texmaker and TeXstudio, because they are quite well ranked in the "Big list", and one of them was born as a fork of the other one.
What are the main differen...
I took the image size reported by pdftex in your original example and replaced by a rule of the same size. I now get the same error so I updated your example and deleted all my earlier comments. this looks to be an error in the right-to-left support babel package which is re-ordering pdf internals and making a "restore" come before a "save" and corrupting the command stack. — David Carlisle4 mins ago
@tohecz I was just going to ask @JosephWright that same question, Joseph do you know if the save/restore pairs in the L3 drivers are safe wrt bidi controls? (Also I haven't quite worked out where in the output routine it trips, it's sensitive to the float size.