The best thing to do to see if you are maxing out your VRAM on your video card is to run an in game performance overlay, or the software that came with you video card (in the background).
4GB *should* be enough to run TS well, even with the amount of rolling stock you have in game. If all of those locomotives/trains in your free roam are condensed in the same area (same tile especially) then your VRAM would see the most use when you initially load that tile.
Below is an example of a test I did a few years ago on Tomcat's Mojave-Cajon route, in the West Colton Yard. This route is very detailed, and uses a fair amount of system resources even before any stock is placed on the track. All 39 locomotives in the scene are idling and 'playable' in this free roam. There are over 900 freight cars in the same tile also.
64bit Test.jpg
Notice that the in-game FPS counter is at 17fps. Then look to my system performance overlay in the top left corner. Only 10% of my video card's 8GB is being used at that moment to display this scene. That is only 800MB of VRAM in use. Now if I were several tiles over, and flew or drove a train towards this tile, that VRAM usage would spike upon the tile loading (usually when you are 1-2 tiles away). If I recall correctly, the highest VRAM usage I saw in this scenario was just over 3GB upon this tile load. Also, the GPU is not even being called upon to run up to full speed.
What does this mean? It means that the extra 4GB that my video card has over yours wasn't used/required by the game in this instance.
So why is the FPS so low? The limiting factor here is how much data can be passed through the computer's CPU at one time. My i7-7700 runs at 3.6GHz, and is no slouch by any means either. Having this amount of running locomotives, in combination with all of the other items being drawn, puts a massive bottleneck on the amount of computations the game is asking the CPU to do.
So in summary, I don't think that upgrading from your 4GB card to an 8GB card is going to give you much, if any boost in FPS. For your own edification, I would still use a performance overlay or data tracking program, that way you can see which part(s) of your system are holding back your performance the most.
You do not have the required permissions to view the files attached to this post.