Many people lay yard tracks, etc on a sheet of cork or something that's the same thickness of the roadbed.  Then there's no need to worry about matching height.  They manipulate the terrain to give the illusion that the yard tracks are at a different level (or not).

You can also sand cork roadbed down into a slope.  Save the cork dust for groundcover.

My father's layout followed the same design as yours: elevated main on cork and yard/engine facility tacked to the deck.  A simple solution is to cut wedges, or "shims", from foam board and stick them under the track from your mainline turnout.  In his case, the grade didn't matter so the track was brought to level in just a few inches (cork is ~1/4" thick which is appx. a 2% grade over 1' of track).  You can adjust yours to be more gradual.

-Johnny

Nah, just kidding.  Nothing wrong with 4x8, if that's what works best in the situation.

First off, I would not recommend laying the track directly on the plywood.  You will find that it is noisy, and some plywood has annoying bumps, wrinkles, etc. which can result in trouble spots in the yard.  Unlike the prototype, where the main line gets the best care and maintenance, model railroads need the absolute very best ever track in the yards, because that's where switching and lots of movements will occur, and where derailments can be hardest to reach or deal with.  So, I would strongly suggest that you lay a sheet of cork for the whole yard.  I actually prefer ceiling tiles, but cork is cheaper.

Of course, there's no reason why you cannot put the main line cork on top of the yard cork, if you like.  The main thing to watch out for is keeping your turnouts level, starting the grade beyond the frog at least.  That means that crossovers really need to be at a consistent level, and that often means that the rest of the yard needs to be at the same level as well.  You might find it easier to use a heavier rail for the main line, along with better, more consistent ties, and a different color and/or size of ballast, all of which would reinforce the idea that that tracvk is different, without needing to have it at a different height.

A short section of track on a shallow grade...

Build a wedge if you are hand laying track.  However, flex track will form the necessary vertical curves without support in the curve area.  Fill underneath with dirt, sand or ballast and glue just like ballasting and only you will know.  The glued material acts as the wedge.

Been using this technique when doing transitions from one thickness of cork to another on my railroad.

Terry

Here is a way to make ramps, in "N" scale anyway.  Noise is not a problem with trains on top of the plywood shelfs.