Exclude one clock from clock tree synthesis

Hello all,

My digital system uses a multiphase clock. Each phase is defined as a independent clock. I have four phases defined in my .sdc file: CK1 CK2 CK3 and CK4. These 4 phases are generated with a custom IC design.

In order to meet the timing requirement, I need to avoid to have a clock tree for CK4. The buffer from the custom IC design will have the sufficient strenght to drive the clock line. Do you know which command I can use to avoid the placement of any buffer on the CK4 line? I'm using Enconter 14.26 and ccopt flow -integration native

Thanks!