Lose the arrival/departure track, you are only going to be able to run one train anyway so you really don't need a A/D track or even a lead.
To amplify on what Dave wrote: on a 4x8, you are not realisitically going to model a biggish yard where you have engine service tracks, a dedicated caboose track, a dedicated arrival/departure track and a number of classification tracks.
You will not be swapping power on trains heading from the plains into the mountains. You will not be building several blocks of cars at the same time, which will then head out on different trains.
What you might be able to fit is a small auxilliary yard of some kind.
Maybe an interchange "yard" - a few tracks where some cars "was dropped off during the night", and where you will be leaving a few cars at the end of your shift, "to be picked up during the night" or some such thing.
Or maybe a small industry yard - where you can temporarily stash a few cars (either empties or loads) while you are switching your industries.
Neither needs to be more complicated that just a few tracks directly off the main.
You certainly don't need a dedicated A/D track or a caboose track if you are modeling a small part of a branch line where traffic is low - you can do any switching directly off your main track.
A couple of examples of small, but functional yards on H0 scale 4x8 layouts, from Byron Henderson's web pages:
http://www.layoutvision.com/id47.html
But I would start by first trying to figure out
what you are trying to model, instead of first drawing a generic "yard" with all kinds of trimmings, and then trying to figure out what it will be used for.
Good luck with your design!
Smile,
Stein