Mike,
A 4 x 16 is a nice size layout. Will it be setting in the middle of the room or against a wall? If in the middle, no problem getting to all of it for track and scenery work. If against a wall, stretching over a 4' table is tough. As many others have responded to you - get a track plan on paper before getting too deep into the table structure. I used 3/8 plywood on a 1 x 4 frame for about half of my layout. The other half was cut-to-shape strips of plywood secured to "T" supports. A sheet of plywood tends to "flatten" the layout.
If you could share your vision of your layout, the builders here could probably help with more specifics. Is your railroad going to be in the mountains, near a river/lake/ocean, out West or in the city, what year are you modeling? You've probably had some "pictures" floating around in your head, try to start detailing them - write down some basic ideas and post it here. What ever you decide, Enjoy - Miniatures are Great!
MIDRANGE - Like you, I got antsy after I purchased a DCC loco and Digitrax Zypher. Had to get some track down!. I dug around and found a small switching yard called John Allen Timesaver. I mounted that on a 2 x 5 piece of plywood on a 1 x 4 frame with the idea of incorporating it right into the main layout when I got it designed. That worked out well - I was able to blow a whistle and let off steam while planning the main layout. Keep us posted.
Welcome to both of you - Mike and Midrange
Regards,
Jon NFL