If you have SampleTank Custom Shop v3.7.3, that is, the free edition, then, yes, it has a small selection of instruments. The paid-for editions have much bigger libraries and a wide range of sounds For example, ST3 XL has over 4,700 instruments.
If you have done the Import(s) and added the ST3 library path to ST4's Settings / Sound Content tab then you should be able to load and play the ST2.5 instruments in ST4.
Let's go back a bit. After the Import(s) can you load and play the imported ST2.5 instruments in ST3?
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The simple way is to buy one of the ST4 editions. You can find the instrument lists in PDFs linked at the bottom of this page.
https://www.ikmultimedia.com/products/st4/#soundsOr you could get Total Studio 2 Max at a bargain price.
https://www.ikmultimedia.com/news/?item_id=9367