Sadly, no - you need a more feature-rich (and expensive) watch/tracker like the Vivoactive or Instinct to connect external sensors. You can use the VS4 to broadcast heart rate data to another tracking app: that was what I did for cycling activity until they added the cycling profile with connected GPS, using the app for my Wahoo sensors, but the integrated Firstbeat tracking/calorie estimations are WAY better than Wahoo's estimation algorithm (especially as I've gotten increasingly fit - standardized algorithms tend to overestimate my calorie burn by quite a lot now), so I switched to using the cycling activity with connected GPS on the watch, while redundantly tracking cadence and speed at the wheel through Wahoo's app if I want that data for a given ride. I haven't tried many other cycling activity apps - there might be one that has comparable energy use estimations and can connect to your sensors (including pulling heart rate data from the VS4 over ANT+), in which case that's going to be your best option, though I don't know if the ANT+ data the watch sends is granular enough for tracking the kind of heart rate variability information that the VS4's integrated Firstbeat algorithm uses.
App-based activity tracking from Garmin that could pull all relevant information from the tracker plus data from third-party sensors rather than handling activity tracking on the watch itself would be great, but that might impact their sales of more expensive watches by allowing similar tracking features as long as one can carry one's phone, so I'm not anticipating they'll do that (I'm a little surprised they added connected GPS at all, since GPS was/is Garmin's primary focus and was a major draw for their more expensive trackers that included it). They apparently used to have one called Garmin Fit, but I think it was retired as they shifted the tracking features to the watches themselves.
