Azul C4 is not a pauseless GC. In the documentation it says "C4 uses a 4-stage concurrent execution mechanism that eliminates almost all stop-the-world pauses."
> C4 differentiates itself from other generational garbage collectors by supporting simultaneous-generational con-
currency: the different generations are collected using concurrent (non stop-the-world) mechanisms
> C4 differentiates itself from other generational garbage collectors by supporting simultaneous-generational con- currency: the different generations are collected using concurrent (non stop-the-world) mechanisms