All four books stand alone. They all have much the same feel and storyline. The read in all four is similar, of the main character's unraveling a mystery. McDevitt writes quite well but the similarities between books were played out for me after Seeker. I probably could have read just the first book and been fine.
He does well to have created a world and present people who think differently. One of the frustrations I had early as a reader was at just how naive his characters must be to fall for some tricks by the bad guys. The answer is that their society had evolved considerably past ours. The crimes familiar to us today were unfamiliar to the characters in this future, just as we'd probably be considered naive if we took our 21st century ideals into the 1500's.
My verdict is to read at least one and just quit any time you want.