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The reason I didn't apply for the H4 extension was (besides being a little careless) that I was advised that the adjustment of status grants an 'adjustee' status until the application is adjudicated upon. Once that was done, all beneficiaries had a dual status (H1/adjustee, H4/adjustee). Applying for an H1 extension maintained my dual status; not applying for an H4 extension meant that my dependant beneficiaries lapsed to just the single adjustee status.
I could well be wrong, but I'd like to believe that I am not. In any case, as I said earlier, it is better to be safe than sorry, and for all those who have the chance, it's probably a good idea to play safe and go for the H4 extension as well. Unless, of course, their lawyer is in strong agreement with the broad line of reasoning I have detailed above.