Which one is correct?
I visited four countries over the period of 2010 to 2014.
I visited four countries over the period 2010 to 2014.
I see more of sentences like "I visited four countries over the period of 4 years", which, I certainly believe, must be the correct version.
First off, 'over the period' means summation of a period of time. So I see no reason someone should section the period of time already merged, especially when the expression has to do with 'over the period'.
Both seem correct to me. But
sounds more correct.