Using the traditional model, the estimates for both are significantly higher. The city's May 2017 cost estimate (0 to 2% design) was $6.8 billion for Relief Line south - and that's strictly the capital, and not including "the cost of potential bus terminals, or maintenance and storage facilities" and "potential DBF costs associated with financing, AFP/procurement". I'm not even sure if it includes property acquisition!
Somehow in the Feburary 2019 Metrolinx report, the cost of Relief Line south has dropped to ±$5.2 – 5.5 Billion. Whether this be grossly out of date, of reflecting an advancement in the design I don't know.
There's no indication it's $4 billion though, or the one-station Scarborough Centre station would be only $3 billion!