To make a Trainz layout/route appear to be a scale model, it is necessary to place assets beside it which are that much bigger than in the real world. For example, to make a virtual model railway to the UK 1:76 scale, since the size of Trainz models is invariable, they have to be compared with artefacts made 76 times larger than in the real world. To model in 1:87 scale, they must be 87 times larger. For your scale, 1:10, they must be made 10 times larger. Although I have uploaded a number of assets for the common model railway scales, I have not made any for for 1:10 scale since this is a unique choice.
To measure the height of a backdrop for a virtual model railway, I suggest placing it on a bare baseboard, then using the height ajdjustment tool to lower it until the top just vanishes. The amount lowered then gives you the backdrops heigh in Trainz metres. Divide this by your chosen scale and you have the model railway height as a metre or fraction thereof.
If I wanted to make a backdrop 18 inches high tor a 1:76 model railway, I woud therefore make it 76 x 1.5 feet high, which equals 114 feet or 35 meters in GMax which I use. (This is a slight approximation to the nearest whole number.)
For your floor to be 4 feet below baseboard level, it should therefore be lowered 40 feet or 12.25 metres (approximate) in Trainz.
You may find this of interest:
A table of model railway scales. (auran.com)
I hope this is helpful - it is certainly intended to be.
Ray