Using content from UE4 Sample Projects

Yes you can.
From FAQ:
"Q: Can you use the content in the marketplace for commercial use ?

A: You can use all of the free Epic content in the marketplace commercially in any games you build with UE4. This includes all of the content residing in example projects. For example, all of the weapons, meshes, sounds and materials in Shooter Game. Tappy Chicken even!"

https://wiki.unrealengine.com/Common_Questions_and_Answers