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.
Both seem correct to me. But
sounds more correct.
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'.