Sorry I missed the Original. Most of these type sci-fi television series start off with a good concept, but work their way into a problem.
The writers never know if there will be a 2nd season. They don't know whether to have a conclusion or set up a cliff hanger. Trying to think positive about renewal they most always choose the latter, but they attempt to keep it flexible (and boring). The later episodes get so uneven they seem to turn to "dumb", lose audience. and we never here from them again.
4400's writers were talented enough to keep the audience involved for 4 years. (The original 'Star Trek' only lasted 3 years, to my dismay. Friday Night broadcasts killed it. Us young adult fans usually were living life on Fridays, then hoping to catch the reruns. No VCR's back in mid 1960's)