Try Riffworks T4 it is free
If you like it you can buy it for around a hundred bucks. building on the timeline you can make individual rifftracks any size you want. 4,8, 16 bars then when you have a few riffs drop them on the timeline in any desirable order.. I use cubase for all mastering and final builds but RW is my tool for brainstorming. REALLY it is the easiet no brainer software out there to use.
RW standard has a few more options than T4 like.. Rewire, rex-player more tracks per riff ect.