The L.A. Daily News indicates that "JP4" is set to begin production some time this year, which means that Mr. Neill won’t be included — mainly because he already has his next two projects locked in, but also because he hasn’t received the call. Regarding earlier reports that Neill’s character would be coming back, the actor dismissed those reports as internet rumors.
And that’s all we got, news-wise. Personally I think it’s a mistake to boot the guy from the series. The actor brings a world-weary gravity to "JP1" and "JP3" that a more "actiony" hero would probably lack. Plus Sam Neill’s just a cool actor, period.
Source: L.A. Daily News