H1b to H4 and back to H1B question

I am currently on H1 B working for a company in the US. I am planning to resign this job and search for a new job.If I dont get new job immediately and if were to change my status to H4, How long does it take to get H4 approval. If the H4 approval takes lets say 3 or 4 months and if I get a new job which is willing to file H1B. Is it valid that an employer can file H1 B over a H4 petition which is still under waiting for approval status?

I recently applied for COS from H1b to H4 and it got approved within 15 days.

However in mean time if u find a job and want to apply for H1b it is possible; you can apply for premium processing… You can make this COS void by traveling out US and stepping back if u have unexpired H1b stamping and newly approved petition. However This is not an expert opinion so Please do talk to attorney for applying back to H1b before you take final decision.