It worked at Keele and Woodbine stations when they were new.
We're not talking about terminating a line here, we're talking about short turning trains. We have to somehow have to coordinate Line 2 trains running from Kennedy to Sheppard-McCowan, and a line 4 spur that runs from STC to west along Sheppard. In essence, we need to somehow turnaround Line 4 trains at STC without heavily conflicting with Line 2 trains that are passing through the station as if it's a normal station.
The point I was trying to get at is that at stations like Glencairn where you can find trains short turning, you can usually find storage tracks of sorts that can be used to store trains in order to have them fit nicely into opposing rail traffic. In the case of Glencairn, trains will drop passengers off at the northbound platform, head to the Lawrence West storage track, wait for the next available slot, and head back out to Glencairn (Source: CartoMetro):
Meanwhile at STC, all we have to work with is this double crossover immediately north of the station (Source: RailFans Canada):
This leaves us with only 3 real possibilities:
1) Terminating Line 4 trains have to cross over to the Northbound platform, and turn around while at the station. This can only really be done if it enters the station immediately after another trains departs the northbound platform, where it will have a little under 3.5m to do a turnaround (The only way this could work is if half of the L2 trains terminate at Kennedy, and I'm also assuming that L2 has ATC and can run at 2m headways, something that this service pattern should be designed around) AND ideally it isn't being followed by another L2 train heading south from Sheppard-McCowan. The problem of course is that this is easier said than done. If that train heading north to Shep is in any way delayed, the L4 train has to sit there in front of the crossover until the northbound train passes, and in that time the next train south could arrive which can create a service knock-on effect.
2) This next option is we make use of the next turnback facility available which is immediately east of Kennedy. In short, this would extend this Line 4 spur south to Lawrence East, and trains will travel to almost Kennedy before using the storage track there. There are 2 potential issues with this: 1) The trains will have to share the storage track with the trains short turning there from the west which would have to be scheduled extremely tightly (assuming it's even possible), and 2) the storage track seems to only have an entrance on the west side, and whether or not it's possible to add track connections to the east, I cannot say.
3) This final option is a modification of Option 2 where instead of having any trains short turn at Kennedy, we instead extend the spur train to go all the way to Kipling. In essence we're creating 3 service patterns: Sheppard-Yonge to Sheppard-McCowan, Sheppard-Yonge to Kipling, and Sheppard-McCowan to Kipling. This is probably the safest option in terms of disruption, but is still likely to be prone to delays as we're now dealing with a mega line where a delay at Bessarion Station (random example) can cause a knock on effect and affect the service at Royal York.
The reality is, none of these options are that good. Any attempt to service STC using the existing Line 2 tracks will result in huge sacrifices to the reliability of both lines, and one must ask whether having a direct connection to STC is worth all of this hassle. This is even before we consider whether it's financially viable to build an infill junction box between Sheppard-McCowan and STC. It's entirely possible that doing so would be just as, if not more expensive than just building a brand new tunnel to STC. If we were to take the logic of "STC is a huge hub, that people would want/need to visit", then the obvious answer is to just build another one at Sheppard-McCowan, and have 2 major hubs surrounding the 401, one served by Lines 4, 2, and the Eglinton East LRT (assuming that gets built), and the other served by Line 2 and DSBRT.