+1, absolutely. After 4 weeks, you're at 44 bpm?! I would sharpen my sticks and impale myself in no time!
What to practice, other than rudiments, depends on you and your goals. I think it's wise to have good command of singles, doubles, and flams, but also consider working on some accent exercises that force you to control your varying stick heights, and practice all 4 types of stroke: free (rebound), downstroke, tap, and upstroke. These ideas are your fundamentals, so when they get easy, push the tempo!
It bears repeating what TB said about the tempo: try a variety of speeds (at least 3!) for every exercise you encounter. What your hands do to perform a task at 40 bpm is nowhere near the same thing as it is at 120. Only when you realize that "hey, at a certain speed, my stick height is too high after this one stroke, and it's keeping me from making my paradiddles get any quicker" will you be incentivized to practice those motions slowly. If you never push the tempo, you'll never find out where your weaknesses are! If you are never wrong, you cannot learn!
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These are all good suggestions. Here's another one that might help--temporarily set aside the single beat exercises and try practicing some of the short roll studies a little further on in the book, or the triplet exercises. When I first started working out of Stick Control I found that the short single stroke roll exercises especially were very helpful in developing my chops. It'll be a change of pace, and perhaps a tiny bit more interesting than what you've been doing.
Or you can experiment with practicing the single beat exercises in different ways. In addition to different stick heights and different tempi (as previously suggested), try swinging them (i.e, playing them as broken triplets). You can also practice them with french grip and focus on using your fingers (or a combination wrist/fingers) to make the strokes. Or play them as 8th note triplets. These are all things that I've done and found to be helpful (as well as a cure for some of the inevitable boredom that sets in from practicing a lot of Stick Control).
One last thought--I bring this up because I've made this mistake in the past: 1) if you're trying to develop your wrists, make sure you're not unknowingly using some forearm to make the strokes; if you do use the forearms (raising them up each time to make the stroke), it will significantly decrease your progress in developing the wrists (and hence it will be difficult to get faster). And 2) make sure you're throwing the stick down each time as quickly as you can (and, since you're trying to develop the free stroke, letting it bounce back up just as quickly). If you're practicing at 40 BPM, I think it can be a common error to just lob the stick down at a slower speed. For the most part, the main difference between playing the exercises at 40 and playing them at 120 is just the lapse of time between the strokes--the actual speed of the strokes themselves should be the same at both tempos.
Ed