Snow meant the stage from Llivia to Igualada was reduced by 50km and after several failed attacks towards the end of the day it came down to a bunch sprint which Bohanni (Cofidis) took comfortably.
He finished ahead of Davide Cimolai (FDJ) and Daryl Impey (Orica-Scott).
BMC's Van Garderen finished safely in the pack to retain his 41-second advantage ahead of the fifth stage which features a 1,000 metre climb towards the finish in Lo Port.
Team-mate Samuel Sanchez is his closest challenger with Team Sky's Geraint Thomas a further three seconds back. Alberto Contador is seventh, one minute 13 seconds in arrears.
Tour de France champion Chris Froome (Sky), Alejandro Valverde and his Movistar team-mate Marco Soler attacked near the summit of the second category Tudo del Puig and managed to open a small gap before being caught with five kilometres left.

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