That's an interesting suggestion, but I'd change two aspects:
1) Would consider changing the type of trains, to something smaller and with less imposing footprint, to reduce the construction costs. If we expect this line to never exceed 15,000 or 20,000 pphpd, there is little reason to spend money on infrastructure capable of carrying 35,000 or 40,000. Still fully grade-separated for speed, with wide station spacng, but not as expensive per km.
2) Don't really like to send it to Finch. The chance to share the TYSSE tunnel between Line 2 and Line 4 is gone. TYSSE could have been built with such sharing enabled, but it wasn't, and now it would be too expensive to connect Line 4. Thus, we would have to duplicate TYSSE and the Line 4 tracks going north-west in the same area, which is kind of low-density even for one subway line. Plus, the Line 4 extension would take decades to plan and fund, while Finch West LRT is nearly shovel-ready and I wouldn't want to stop it now.
Instead, I would connect Line 4 to the Spadina line at Wilson rather than at Sheppard West (former Downsview), then extend it further west along Wilson. West of Jane, it could swing to the 401 corridor and fast-track to Pearson (connecting to the Etobicoke North RER station en route).
That would be a faster connection to Pearson than going via Finch and Humber college. The northern route would be served by Finch LRT, while the Sheppard-Wilson crosstown line would fill the gap between Eglinton and Finch.