Yeah, working on it.
Too much to do, not enough free time, and collaborating with several other folks can really slow things down as it can take a day or two (or more) to get notes back from others.
Short answer, 150 Karma. You can opt to Promote a character to Prime Runner status at any karma point, but going into Season 4, if you have 150 Karma or more, you must Promote your character to Prime Runner status and create a new one.
There will be some minor perks that your new character will get (Higher karma characters will provide more perks, a "thank you" for active Missions players), and Prime Runners can be played in special Prime Runner adventures and in Special Event Missions (Things like the Scrambles and events like Appetite for Vengeance, the Holiday Charity Mission). For Seasonal adventures (SRMs) and Convention missions (CMPs) you'll need o start a new character.
Like season 2 to 3, Karma Transfer and Character transfer options are available for characters under 150 Karma.
I would request players not deliberately cheese the system and get as close as possible to 149 before starting Season 4. One of the points of the Promotion System is to help rebalance characters a little bit.
And, because this will likely be asked, this point total will be re-evaluated and probably tweaked when we move into Season 5, based on how many adventures we release during Season 4, the average Karma we're awarding (Which will be changing in Season 4), and the like.
Bull