Sofia Metro Line M4 (Obelya – Sofia Airport)
The yellow line, 23.0 km with 20 stations, from the Obelya estate across the city to Terminal 2 of Vasil Levski Sofia Airport. For most of its length it has no tunnels of its own: between Slivnitsa and Mladost 1 it uses the line M1 tunnels built between 1998 and 2009. It is a branch in its own right only at the ends, in the 1.8 km Obelya spur opened in April 2003 and the eastern extensions to Tsarigradsko shose (2012) and the airport (2 April 2015). Trains through-run at Obelya onto line M2; until 26 August 2020 the airport service was signed as part of M2, and it was renumbered M4 in preparation for the infill station Pancho Vladigerov, which will physically separate the two lines and let them keep independent timetables. Since 19 July 2025 M4 has terminated at Slivnitsa while that station is built.
- Total length
- 24 km
- Open today
- 24 km
- Sections
- 4
- Cost on record
- €90.584M
- 1 of 4 sections carry a comparable figure
Restated into EUR at 2025 prices.
Includes 14 km whose works another project already measures, either as shared track or as a structure inside one of its sections. Counted in full here, and once in any total that spans projects.
Sections
| Sections | Status | Length | Timeline | Cost | Funding | Contractors | Contract terms |
|---|---|---|---|---|---|---|---|
| Obelya spur: Obelya – Slivnitsa 1.8 km branch from Slivnitsa to the Obelya housing estate and depot, built as an extension of line 1 and opened on 20 April 2003. The infill station Pancho Vladigerov is under construction on this section; M4 has terminated at Slivnitsa since 19 July 2025 | Opened | 2 km |
| – | – | – | – |
| Slivnitsa – Mladost 1 (shared with M1) Shared track Shared with Sofia Metro Line M1 (Slivnitsa – Business Park Sofia) M4 trains use the line M1 tunnels between Slivnitsa and Mladost 1, opened in four steps between 28 January 1998 and 7 September 2009. No length is published for the shared section on its own; 14.2 km is measured from the OpenStreetMap centreline and is consistent with the published 23.0 km line total | Opened | 14 km |
| – | – | – | – |
| Stage II, Lot 2: Mladost 1 – Tsarigradsko shose Lot 2 of the second construction stage: 2.5 km and two underground stations, Mladost 3 and Inter Expo Center – Tsarigradsko shose. Works began 15 February 2009 and the section opened 25 April 2012 | Opened | 3 km |
| – |
| – | – |
| Stage III, Lot 1: Tsarigradsko shose – Sofia Airport Lot 1 of the third construction stage: 5 km and four stations (Druzhba, Iskarsko shose, Sofiyska Sveta Gora, Sofia Airport), two underground and two on the surface, reaching the Terminal 2 departures gate. Started 2013 and opened 2 April 2015; the construction contract was €69,923,066 excluding VAT | Opened | 5 km |
|
|
| – | Contract value €69.92M |
This section runs on track another line already owns. It counts in this line's length and is left out of every total that spans projects, so the kilometres are counted once.
Funding
- European Union8 km2 sections
- Cohesion Fund grant of €200 million for the second construction stage, shared with the M2 section from Obelya to the Nadezhda overpass
- OP Transport 2007–2013, Priority Axis 3: ERDF grant of €93 million towards the €110 million third construction stage, which also covered the M1 extension to Business Park Sofia
- National budget5 km1 section
- Loan3 km1 section
- 20% local co-financing by Sofia Municipality from a European Investment Bank loan
A section that lists two sources counts its full length under each: the data records which sources paid, not what share each one carried.