Its been 4 weeks since i mailed in my return and irs still dont have it..what shall i do
It's very possible that they may have it but just not started reviewing/processing it yet. Until they do, you won't find it in their system when you try to track it. Allow them a couple more weeks and then call them. Mailed returns always take a bit longer to get entered in. If you have to ever mail one again, you may want to consider using a tracking service with USPS or maybe FedEx so that you know once they receive it.
You only want to call them if it’s been at least 6 weeks since you mailed and at that time, you can reach them at the IRS tax refund hotline at 800-829-4477, Monday - Friday, 7:00 a.m. to 11:30 p.m. Or, call the 24-hour assistance number at 800-829-1040
If you mailed state, choose your state here How do I track my state refund? and it will provide you instructions on how to check the status of your state return on your state's website.