By Nigel Wynn
It's the first time that Martin has ever worn the yellow jersey of Tour race leader - something that he had been tantalisingly denied on the preceding three stages, missing out on Monday by just one second. Canny riding over the cobbled sectors and a well-timed attack saw the German former world time trial champion finally realise his dream.
John Degenkolb (Giant-Alpecin) brought the chasers home for second place, with Peter Sagan (Tinkoff-Saxo) in third.
Martin takes the yellow jersey from overnight leader Chris Froome (Team Sky), who finished safely in the pack along with fellow contenders Vincenzo Nibali (Astana), Nairo Quintana (Movistar), Alberto Contador (Tinkoff-Saxo) and Tejay van Garderen (BMC).
The much-anticipated stage of the 2015 race did not disappoint, with seven cobbled sectors in the second half of the stage provided plenty of opportunity for attacks and mishaps - although the main contenders neither gained or lost time on each other.
The day started calmly in Belgium, with escape group regular Thomas De Gendt (Lotto-Soudal) part of the day's break along with Lieuwe Westra (Astana), Perrig Quémeneur (Europcar) and Frédéric Brun (Bretagne-Séché Environnement). The four riders managed to stretch out an eight minute gap quite quickly as the peloton was content to let them go over the first half of the day's action.
Mark Cavendish (Etixx-QuickStep) led Martin at the head of the peloton through first of seven sectors of pavé, Pont-à-Celles à Gouy-lez-Piéton - at which point the break's advantage had been pegged back to just 1-10.
Just after the first pavé sector, Contador swapped his bike for his cobbles-ready Specialized machine - a tactic that several other teams also employed to deal with the 13km of rough roads.
There was a brief lull in pace at the half-way point of the long 233.5km route, and the break's gap stretched out to over three minutes again as there was a lull in the action as the 2015 race entered France for the first time.
With 40km to go, the break was caught and the pace picked up as teams jostled for position at the front of the bunch after the third of the seven sectors of cobbles. Just as he did last year, Nibali looked very strong on the rough roads - riding right at the front of the bunch and setting the pace.
Relatively dry conditions helped to keep the lead group together, and a sizeable peloton hit the final cobbled sector together. Thibaut Pinot (FDJ) suffered a badly-timed puncture and lost contact with the leaders - and appeared to lose his cool, too.
By the end of the final pavé sector and with 10km to go there were eight riders in a lead group, including Froome, Nibali, van Garderen, Degenkolb and Alejandro Valverde (Movistar). With no one wishing to do more than their share of work, that group was caught by the chasing pack.
Martin then attacked with 3km to go, slipping into time trial position and opening up a large gap, which he managed to retain until the line in Cambrai.
Martin now leads Froome by 12 seconds overall, with van Garderen in third at 25 seconds. Despiet losing the overall lead, there will be an element of relief for Froome and Team Sky as the burden of race leadership and media attention turns to another rider and another team.
British time trial champion Alex Dowsett (Movistar) was brought down in a crash, but managed to finish the stage alone and as last man despite a deep laceration to his right arm that subsequently required stitches.
The race should calm down slightly on Wednesday for stage five, which on paper looks like a day for the bunch sprinters. Starting in Arras, the route takes a relatively flat path over 189.5km to Amiens.
Tour de France 2015, stage four: Seraing to Cambrai, 223.5km
1. Tony Martin (Ger) Etixx-QuickStep in 5-28-58
2. John Degenkolb (Ger) Giant-Alpecin at 3 secs
3. Peter Sagan (Svk) Tinkoff-Saxo
4. Greg Van Avermaet (Bel) BMC Racing
5. Edvald Boasson Hagen (Nor) MTN-Qhubeka
6. Nacer Bouhanni (Fra) Cofidis
7. Jacopo Guarnieri (Ita) Katusha
8. Tony Gallopin (Fra) Lotto-Soudal
9. Zdenek Stybar (Cze) Etixx-QuickStep
10. Bryan Coquard (Fra) Europcar all same time
12. Mark Cavendish (GBr) Etixx-QuickStep at 3 secs
13. Rigoberto Uran (Col) Etixx-QuickStep at 3 secs
15. Vincenzo Nibali (Ita) Astana at 3 secs
17. Chris Froome (GBr) Team Sky at 3 secs
19. Alberto Contador (Spa) Tinkoff-Saxo at 3 secs
22. Geraint Thomas (GBr) Team Sky at 3 secs
23. Nairo Quintana (Col) Movistar at 3 secs
24. Tejay van Garderen (USA) BMC Racing at 3 secs
Overall classification after stage four
1. Tony Martin (Ger) Etixx-QuickStep in 12-40-26
2. Chris Froome (GBr) Team Sky at 12 secs
3. Tejay van Garderen (USA) BMC Racing at 25 secs
4. Tony Gallopin (Fra) Lotto-Soudal at 38 secs
5. Peter Sagan (Svk) Tinkoff-Saxo at 39 secs
6. Greg Van Avermaet (Bel) BMC Racing at 40 secs
7. Rigoberto Uran (Col) Etixx-QuickStep at 46 secs
8. Alberto Contador (Spa) Tinkoff-Saxo at 48 secs
9. Geraint Thomas (GBr) Team Sky at 1-15
10. Zdenek Stybar (Cze) Etixx-QuickStep at 1-16
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