So there is a hard limit that is in place by default which restricts your landscape from using more than 4 texture samples per landscape component. To avoid this limitation you can set your texture samples to Shared:Wrapped which will allow you to use up to 128 unique texture samples.
This was implemented in our 4.6 engine version release which you can look over for yourself by following the link below.
[Unreal Engine 4.6 Release Notes][1]
Let me know if you have further questions or need additional assistance.
Cheers,