My 4 year old has 8 cavities. Should she be given sealers?

No. Sealants are placed on teeth that do not currently have cavities and are considered a preventative treatment. Once a tooth has been diagnosed with decay, it must be restored with a filling or crown.
Sure . Sealants are applied to non decayed teeth to prevent cavities so its too late for the teeth that already have them. However, those teeth that do not have cavities or are erupting into the mouth can benefit from sealants. I would consult with your dentist to see if such teeth exist.