LONDON (Reuters) - Oil fell more than $4 a barrel to below $121 on Tuesday, touching the lowest price since May, as signs of weakening demand outweighed a disruption to Nigerian oil output. The drop also coincided with a firmer U.S. dollar, which may have reduced the appeal of commodities to some investors, and comments from OPEC's president that oil could fall to $70 or $80 in the long term. "We still believe that crude's rallies are vulnerable and we would advise not buying into them," said Edward Meir, analyst at MF Global who earlier on Tuesday said he expected...