I called a parts dealer for my car and I can buy the steering wheel controls with the answer/end buttons and voice control button then install that in my car rather easily. My concern is that these SWC were not compatible with my factory radio. Are the answer/end call and voice button compatible with the NEX4000 using the Metra steering wheel controls unit?

Also, if I do buy these controls, will the voice button work to launch Siri if i'm in CarPlay? What about if i'm connected using Bluetooth?

One major problem here is most 3rd party SWC units are protocol converters from OEM to Pioneer, based on make/model of both. Pioneer's is more "standardized" than OEM.  If the OEM SWC was not compatible with your OEM head unit, it's hard to say what is there.  Adding the steering wheel part(s) is no guarantee that rest of the "control" network(s) is there.  If not, there is probably a way implement a custom solution... but you'll need to verify what else is there first, or go complete custom from the get go.