That wouldn't work without the Relief Line to Sheppard, since it would force riders to use Line 4 -> Line 1 instead of Line 2 -> Line 1, which will put more people on Line 1 and those to would have not used Line 1 (destination on Line 2) would need to double transfer Line 4 -> Line 1 -> Line 2.
As a side note, I've always wondered why anyone would want to take Line 2 then transfer to Line 1 to get to downtown if the Relief Line went up to Sheppard. What I would do if I was at STC would be to take the 95 York Mills/Ellesmere to Don Mills, then transfer to Line 3 (DRL) to downtown. It would be even better if it was the 95E bus. The Relief Line will (should) have less stops than both Line 1 and Line 2, which would allow for a faster trip. Of course, this is assuming there's no interlining of trains from Line 2 to Line 3 and vice versa. Please correct me if my perspective on time is in the wrong place