The base tag for the store is: [@warehouse_4@] and I need the stock quantity from this particular warehouse
You can do that with [%warehouse_qty%]:
https://developers.neto.com.au/documentation/neto-designer-documentation/b-se-tag-library/function-tags/product-information/warehouse_qty/
You'll just need to filter it to a particular warehouse with something like this:
[%warehouse_qty id:'[@sku@]'; showall:'1'%]
[%param *body%]
[%if [@warehouse_id@] eq '4' %]
[@warehouse_qty@]
[%/if%]
[%/param%]
[%/warehouse_qty%]