Line 3 is expected to cost around $1.4bn to build in total.
The latest contracts tendered cover the civil works section, encompassing tunnelling and construction of the stations along the route. The estimated cost of this is $700m.
An official at the Tehran Urban & Suburban Railway Company, which is overseeing the metro project, says a small section of Line 4 could open as early as the spring. Although it is only 3 kilometres long and will include three stations, it will link to Line 1 giving easier access to the important retail and commercial district around Enqelab Street.
The metro authority is also purchasing two tunnel boring machines for up to $30m each to assist with its expansion programme.