I did this same thing before I got my Etys for this same purpose but really didn't find it helpful. The ER-20s do have the exact same tips as the ER-4s, but the plastic stem in the middle of the 20s is much shorter than in the 4s, which completely changed the fit for me. I can wear the ER-20s for hours, but I can't stand the rubber tips on my ER-4s for more than about 90 minutes...I have to use the foam tips if I want to use them any longer than that.
I do like the ER-20s a lot though...one of the nicest things is that you can still hear quiet sounds. My wife can speak to me in a normal conversation voice or even less than that, and I can hear her perfectly. However, louder sounds are tamed down a lot which really makes me feel better about my hearing. I play trumpet and our music department's practice rooms are very small, so these really help a lot when I'm practicing. I'll play for an hour or so and then just before I'm ready to leave I'll pull the plugs out and somehow it still shocks me every time I hear how loud it is without the earplugs in. I used to wear them for marching band gigs too, and they were wonderful. It's literally painful to stand in a 44-member trumpet section with the drumline right behind you without hearing protection. The 20s make it totally comfortable.
I would highly recommend the 20s as earplugs, but I would exercise caution in thinking that being able to wear the 20s comfortably means that you'll be fine with the rubber tips for the ER-4s.