Timesplitters 4 because they don't know what they want to do with it yet. Crytek UK is what Free Radical was renamed after the acquisition of the developer who worked on the Timesplitter series. The IP was acquired along with the developer by Crytek.
“Obviously Timesplitters, historically, was for them (Crytek UK) an important IP,” Crytek co-founder Avni Yerli has said. “From our perspective we always had a lot of fun playing Timesplitters – Timesplitters 2 was amazing fun.”
It seems that at least Timesplitters 4 is an inevitability; Crytek has made it clear that something will be done with the IP. According to Yerli, “Crytek and Crytek UK are interested in the IP; we are thinking about it but we’re trying to decide what we could do best with the IP, where we could bring it. There are no concrete plans yet.”
Crytek has also not ruled out HD remakes of the previous games in the series.