I almost know the reason why I could not apply the ECP only to C4: any tag in the ECP directive will try to match as many tags in the geometry directive as possible. Although there is not an exact match for C1, but the C4 tag in the ECP directive has already defined an ECP for the carbon element, so the ECP is also applied on C1.
Maybe the only way to circumvent this is to modify the source code.