Murray is definitely long for his age. He measures a whopping 3 cm (or 30 mm) in length. According to the U. of Minnesota’s Field Guide to Monarch Caterpillars, fourth instar caterpillars should measure from 13 to 25 mm in length.
Fifth instar caterpillars should measure from 25 to 45 mm. His front tentacle measures 10 mm which is the right length for the fifth instar.
So which instar is Murray?
He definately exhibits those white markings on his legs as you can see in both photos. (Click on them to see close-ups.) I have found him walking up the side of the jar so is he seeking a place to pupate? The length of his body and front tentacles are pretty long. that all seems to add up to him being in the fifth (not fourth) instar.