I've enjoyed all the Titan books, and consider Orion's Hounds
the best of the bunch, so you might also want to take my advice in that context. That said, if your problem with Book 3 was a lack of action (and I can't imagine why, given the awesome scenes like the crystaline entities' attack on the refugee station), then you'll probably enjoy Book 4 more, as it is more action-oriented and has a much more clipped pace. Once you get past the first few chapters, shit hits the fan and the hits keep coming.
Fictitiously yours, Trent Roman