wvtrumpet,
I look at this in a different way than most, if you really like running your trains with remote control, I suggest you save your self some money in the long run, and invest in
a full DCS package, along with the side receiver for your Z4K. No matter which engines you have right now you can at least run them in Conventional mode with the full DCS unit. Sense you already have the Z4K the side receiver opens a few more options in the conventional engine mode. Now if you are looking to keep things on the inexpensive side, and only use conventional engines, the Z 4000 remote is the way to go. However if you intend to invest in either MTH P2/P3 or Lionel Legacy engines invest in the full DCS unit from the get go. Later then you can add the 990 Legacy base unit. If you decide you need even more power the old ZW's run the DCS and Legacy perfectly when set up correctly.
Have fun with your new remote control trains!
PCRR/Dave