We have been waiting for Budapest M4 metro line for quite a few years before it finally made it through and it was inaugurated in 2014. Its official name is South Buda–Rákospalota (DBR) Line, Metro 4 or M4, and unofficially people do call it Green Line (the other previous ones being yellow (M1), red (M2) and blue M3).
The construction work eventually started in 2004, and the first section with 10 stations opened ten years later, in 2014. Compared to the older lines it is obviously much more modern, but just like the other lines, every station is different one from the other. Basically, the majority of the stations are decorated with mosaics or are modern-industrial-themed stations which if you are into modern architecture, is great! I personally really like them and the materials that have been used, mainly steel and concrete, are greatly surrounded by wood and glass, giving a hint of more naturalistic feel to the stations.
Differently than the older lines, these new metro stations are very wide and there is a lot of space and height in each of them so you do not feel claustrophobic which is always pretty cool. They dag under the already existing metro stations, this allowing the new ones to be wider and in accordance to the security laws.