The first problem is that a 4 mm is less than the thickness of three pennies stacked on top of each other. An eyepiece with this diameter is so small you will have great trouble looking though it. 10-20 mm eyepieces are usually more popular choices. These provide good magnification while also being easy to look through. Another problem is resolution.
Resolution is the ability to make out detail and is determined by how big across the telescope is.
The greater the diameter the better the ability to make out detail. A magnification that is too great for the telescope's resolution will result in the image breaking down. This is like enlarging a newspaper picture on a copier, taking the enlarged picture and enlarging it, taking that enlarged picture and enlarging it, and so on. Eventually, you reach a point where the picture is no longer visible and all you have are black dots. Finally, remember you are looking though the Earth's atmosphere and that will ultimately control the limit of magnification you can use.