They can continue as long as the I-94 is valid
If the old I-94 is still valid (i.e. your previous H1 has not expired), then it is fine. However, it is better to keep the H1 and H4 in sync just to be on the safe side so that you don\'t forget to apply for the H4 extension before it expires.
Many times is happens that since your current H1 is not going to expire soon, you overlook the fact that your spouse\'s H4 has a different expiry date than your H1 and forget to apply for extension. Also keep in mind that it takes anywhere from 3-6 months for the H4 extension to come through if applied separately (i.e. not with the H1 application).
Hope this helps.