We need to obtain the official government receipt notice back from the USCIS before your current status expires, and the government can take up to 3-4 weeks to issue the notice. Therefore, the 4-week window is least amount of time we need to ensure that we receive the receipt notice back before your status expires.
Note that if your current H-1B status expires while the extension is pending with the government, this is not a problem. The law accords you with an additional 240 days of work authorization past your current expiration date while the extension of status is pending with USCIS. The government receipt notice that we receive from the USCIS once we file your extension allows us to keep you on payroll for the additional 240 days.