Why i cant?
The Intel Core 6400 is only a dual-core processor and a quad-core is recommended in order to run UE4. Below are the recommended specs.
For developing with UE4, we recommend a desktop PC with Windows 7 64-bit or a Mac with Mac OS X 10.9.2 or later, 8 GB RAM and a quad-core Intel or AMD processor, and a DX11 compatible video card. UE4 will run on desktops and laptops below these recommendations, but performance may be limited.
Please feel free to look at this Forum post to see what others are using to run the editor and discuss your specs.