Landa had attacked from the reduced peloton on the final climb on the race's key mountain stage to claim the victory ahead of Rafal Majka (Bora-Hansgrohe) and George Bennett (LottoNL-Jumbo) in third. It's Landa's first victory of the season, and his first for Movistar.
Thomas had a problem with his bike just as the action started to hot up on the way to Sassotetto, and lost valuable time waiting for a replacement machine. He finished the stage 40 seconds down on Landa, and lost the race lead to former overall leader Damiano Caruso (BMC).
Caruso now leads Michal Kiwatkowski (Team Sky) by one second overall, with Wilco Kelderman (Team Sunweb) in third at 11 seconds, with Landa in fourth at 26 seconds. Thomas slips to fifth place at 26 seconds with three stages remaining.
How it happened
The 'queen' climbing stage of the 2018 Tirreno-Adriatico started with an escape group of six riders leaving the peloton.
For the third consecutive day, Nicola Bagioli (Nippo-Vini Fantini) was part of the break as he chased mountain points to extend his lead in the KOM classification and Jacopo Mosca (Wilier Triestina-Selle Italia) hunted for sprint points. They were joined by Krists Neilands (Israel Cycling Academy), Mads Pedersen (Trek-Segafredo), Antoine Duchesne (Groupama-FDJ) and Alexander Vlasov (Gazprom-Rusvelo).
After 60km of the 219km stage, the break had six minutes over the peloton and the gap then wavered between four and six minutes for the majority of the stage.
Disaster struck for Giro d'Italia champion Tom Dumoulin (Team Sunweb) as he was involved in a crash and was forced to abandon the race after starting the day in ninth place overall. His team later reported that he had suffered abrasions, but no fractures.
Team Sky were shepherding the peloton for race leader Thomas, with the team's Jonathan Castroviejo doing some long turns on the front to keep the break in check.
As the race hit 25km to go, the break still had four and a half minutes in hand – but, of course, with the long final climb to come this was not going to be enough. Mitchelton-Scott started to move to the front of the bunch, followed by Movistar, and the chase really started to hot up.
Mosca was the first to drop out of the break after he took the maximum intermediate sprint points just before the final ascent. And heading into the final 10km, road race world champion Peter Sagan (Bora-Hansgrohe) dropped out of the back of the peloton.
Meanwhile, Neilands attacked from the break to go solo up front chased by Vlasov as the rest of the break got mopped up by the bunch.
Astana were at the front of the peloton, with Sky just behind them, and they eventually caught Neilands and Vlasov with 7.3km to go.
Miguel Angel Lopez (Astana) was the first to break the deadlock, attacking from the front of the peloton with 5.5km to go. This was followed shortly after by a move from Fabio Aru (UAE Team Emirates), Majka and Ben Hermans (Israel Cycling Academy). The four joined together with 4.3km to go.
Majka looked in aggressive mood, and attacked his fellow escapees. Meanwhile, Landa attacked out of the peloton with 2.5km to go in an attempt to bridge over to the leaders.
Chris Froome (Team Sky) was one of those lost off the back of the peloton into the final 2km, as Landa joined up with Majka, Aru and Hermans up front after Lopez sat up due to cramp.
Landa evidently wanted to press on, taking to the front with Majka, Aru and Hermans fighting to stay on his rear wheel and with a handy 24 second gap over the chasing bunch with 1.5km to go.
Thomas suffered a badly-timed mechanical and was forced to wait at the side of the road for a replacement bike and could only watch his rivals ride away up the road. However, Froome appeared just at the right moment and played super-domestique for Thomas to try and pull him back up into the action.
As Landa put in one last attack to take the stage victory, Froome had left Thomas, with the latter doing all he could to try and minimise his time loss.
Bennett had attacked from the chasing group of riders to get in touch with Landa and Majka up front, but Landa's final kick to the line left his rivals with no answer. Majka was left shouting with disappointment as the crossed the line, with Bennett's huge effort netting him third.
The 2018 edition of Tirreno-Adriatico continues on Sunday with stage five, a lumpy route from Castelraimondo to Filottrano covering 178 kilometres. The race concludes on Tuesday.
Tirreno-Adriatico 2018, stage four: Foligno to Sarnano Sassotetto, 219km
1. Mikel Landa (Esp) Movistar, in 6-22-13
2. Rafal Majka (Pol) Bora-Hansgrohe
3. George Bennett (NZl) LottoNL-Jumbo, at same time
4. Fabio Aru (Ita) UAE Team Emirates, at 6 secs
5. Ben Hermans (Bel) Israel Cycling Academy
6. Tiesj Benoot (Bel) Lotto-Soudal
7. Romain Bardet (Fra) Ag2r La Mondiale
8. Wilco Kelderman (Ned) Team Sunweb
9. Adam Yates (GBr) Mitchelton-Scott
10. Rigoberto Uran (Col) EF Education First-Drapac, at same time
General classification after stage four
1. Damiano Caruso (Ita) BMC Racing, in 17-14-49
2. Michal Kwiatkowski (Pol) Team Sky, at 1 sec
3. Wilco Kelderman (Ned) Team Sunweb, at 11 secs
4. Mikel Landa (Esp) Movistar, at 20 secs
5. Geraint Thomas (GBr) Team Sky, at 26 secs
6. Rigoberto Uran (Col) EF Education First-Drapac, at 31 secs
7. George Bennett (NZl) LottoNL-Jumbo, at 33 secs
8. Davide Formolo (Ita) Bora-Hansgrohe, at 34 secs
9. Tiesj Benoot (Bel) Lotto-Soudal, at 36 secs
10. Domenico Pozzovivo (Ita) Bahrain-Merida, at 41 secs
Thank you for reading 5 articles this month* Join now for unlimited access
Enjoy your first month for just £1 / $1 / €1
*Read 5 free articles per month without a subscription
Join now for unlimited access
Try first month for just £1 / $1 / €1