All of the commercial-band class Bs in the market are at Empire except 94.7, which is across the Hudson in New Jersey. So is WQXR (105.9B1). 4TS is home to the main transmitters of WBGO (88.3B1), WKCR (89.9B1), WNYE (91.5B1) and a booster for WFMU (91.1). It also houses auxes for roughly half the Empire FMs.
On TV, 4TS is home to WNYE (RF24) and a bunch of auxes.
ESB is somewhat preferred to 4TS, wherever possible, because it's a few hundred feet higher and less surrounded by similar-height buildings that can cause multipath. But the mast space at Empire is quite limited and can't easily handle directional antennas, and that's where 4TS is useful. (It's also useful during blackouts, since it has much more auxiliary power capacity than Empire.)