BSIM4 is a modeling language for accurately characterizing MOSFET devices. It's not a part, but more a language used to describe FETs built on <100 nm processes. BSIM stands for Berkeley Short-channel IGFET Model. Think of it like a SPICE model for a transistor, but with more accurate results.
What you're probably asking for are BSIM4 model parameters for some FET you want to model with ADS. For that, I'd hit the manufacturer's website and look under Development Tools, or Design Help sections, looking for product models.