So I think I have observed a pattern in the behavior of the multiplier. It seems that it goes up a fixed amount every game loop, but goes *down based on time* -- i.e. -- if the browser is running slow, you want to keep user progression, so you calculate the "missed frames". And you drop the multiplier down for the longer time period, but only add in *one frame* worth of addition, rather than as many frames were skipped.
Repeat by: Do something that causes browser lag / graphical update issues while in the 4x zone. The increased rendering takes longer, and it slows, and your level drops. And at the 3x zone, the rendering is faster, and it catches up.
EDIT: I found a way to get this exact same behavior when the tab is "backgrounded" -- without any clicking or window activity, when I bring the tab to the front, the bar (which had dropped to the middle of 3x) climbed back up. So yes -- it seems to be "subtract based on time since last cycle, add one unit regardless of time."