The reason I mentioned the time may be less than this by a matter of minutes was down to differences in displacement at objects moving at different speeds which can be expressed best by the following simple equation
(2.1c)(2.1d)(2.1e))
This relates to the basic linear dynamics regarding the process of travelling at speeds which affect the actual passage of time on differing vessels. For eg, if someone was to travel in excess of the speed of light , he could theoretically return to earth after a few minutes and the time elapsed for everyone else could be months/years depending on the speed travelled. I have assumed all things are equal in your initial question so answered 4 years 6 months but the time would in reality be slightly less assuming the perspective of time elapsed was not from someone on either satellite.