Since the new series 5 is a very minor incremental upgrade from the 4, they discontinued the latter, but still sell the 3. This way the comparison table between 3 and 5 make it seem like the 5 was a bigger improvement.
That's not their reason for discontinuing it though - there's no reason for them to sell the 3, 4, and 5. Probably retooled the 4's assembly line to make 5's.
