Just posting my opinion.
How would I fix Line 4?
1. Convert the line from heavy rail to light metro.
2. Extend it east to STC
3. Maybe extend it to Sheppard West? It would be kinda dumb if line 6 was going to be extended west to Finch Station, let me know what you think of that.
1. I agree, that's probably a better deal than extending it as the present-day heavy rail subway. A more agile option will permit going further east and west, and thus improve the ridership.
2. Not sure if it should connect to Line 2 at STC, or at the Sheppard & McCowan subway terminus. Both options have their pros and cons.
3. Extending to Sheppard West makes sense, and maybe even further west. I would consider flipping from Sheppard West to Wilson at some point, as it will be much easier to organize the feeder services to the terminus on Wilson; the buses could come from Weston Rd North, Albion Rd, and the western 401 / 409.
I would want to see Line 6 coming to Finch & Yonge, notwithstanding the Sheppard line's western extension. Line 6 is of a more local nature, but to avoid unnecessary transfers, the riders should have a direct access to Yonge, rather than having to switch to a bus at Keele, or navigate via Lines 1 and 4.