I've had INR readings at 4.2 and higher. At one time, I even had one around 7 (according to a doctor I used to see). It could be that the blood was mishandled, it could be that my then irregular testing actually enabled me to let my INR get out of control).
I use two or three different meters for testing, and one meter may say 3.2 while another says 4.2 (or higher). I don't really let it trouble me to be in the low to mid 4s. At 4 and higher many clinics will get a blood draw to confirm the value.
The easiest way to deal with a 4.2 is probably to add some greens to your diet. Also - try to stay away from sharp objects and to avoid impacts that can cause bruising. In my experience, a 4.2 isn't that big a deal as long as I'm a bit more cautious than usual. When it gets to 5, 6, or above THEN you should confirm with another meter or a blood draw, check with a doctor, and probably consider taking some Vitamin K supplements to bring it down fairly rapidly. A 4.2, though, doesn't really concern me too much.