Something similar happened with me also in the initial days. You should really check and see in which conditions it is happening to you, kind of root cause analysis

Elizabeth is right, it can be because sometimes you are so obsessed with the preparation that you forget that sometimes taking a break can be even better than any other therapy of the world.
I spent lot of time in gardening or doing other things in the house, helping my wife with her day to day work. You can also talk to your friends and try get relaxed and detached from PMP for a day or two.

You can also do as I did. I never studied on the day when I took my practice exams. Usually I did my practice exams from morning 8 , 9 or 10 so that I am fresh for the exam.

You can also try doing meditation or listening to your favourite music. These things helped me.

If your issue is "being focused for 3 to 4 hours at a stretch." Try this, initially practice 30 questions, then 60 questions, then 80 questions, even better would be doing 60 questions - break of 15 minutes - 60 questions again - a break of 15 minutes - last round of 60 questions. Basically do a slow build up for the grand practice exam.

Hope it helps.

It helps me to read the question intentionally, out loud (although I know in an exam setting you aren't allowed to move your mouth with the online invigilators). Perhaps you simply need a break from studying for a bit?

Hey Guys, recently I have been having problems focusing during the Practice Exams e.g
I am reading a question and I go back to the first letter over and over and it wastes my time spending minutes on a single question, after all that I realized the question was quite simple. How do I prevent this from happening again