If all you really want is the space occupied by a sheet of plywood (no judgement here...just determining a fact), try to keep the plan simple and reliable. Avoid trying to shoehorn another 24" of spur or another siding.
A 4X8 is a good first step as a learning tool and as a stepping stone to discovering what type of railroad you really want. I always encourage people to try for more, but it is risky. A person can get bogged down by indecision when confronted with many suggestions for re-thinking this and considering that.
It seems that the hobby is expensive at the outset, and part of that expense is the original mistakes we make in purchases and in decisions about what to use and how to use it. For example, a train is not a train in the hobby. Some models are better than others, and we learn that some track plans are better than others. It's just that getting to those realizations can set us back about $400! And if we are truly unlucky, about a year of modelling.
I can't in good conscience end without stating that you are sure to outgrow a slab Plywood Pacific before many weeks have passed. As such, I urge you to not glue anything permanently. Try different track arrangements. At some point, you may accept that you can cut the plywood into three pieces, maybe four, and get an entirely different and interesting track plan with more room for other things in the same space.
Good luck.