To my knowledge, there is no time restriction set by the IRS other than all documentation is likely needed to be provided to you before the April 15th deadline. That being said, while 4-6 weeks may be a long time, they are likely basing this off of the time it would maybe take to be prepared and mailed to you.
I would recommend contacting your employers' payroll department and see what options they have to help expedite the process. In the meantime, you may also wish to consider filing an extension just to be safe. For steps on how to do this, please follow the instructions shown in the article below:
https://ttlc.intuit.com/replies/3301811