The antenna will do better if mounted up, in the clear... but there is no harm in trying it in the attic. Metal roofing, ducts, foil faced insulation, etc. will all work against you.
The splitter you linked to has more than the needed bandwidth, and should be fine. Any splitter rated to 1000 MHz (1 GHz) is fine.
It sounds like you will run the new RG-6 from the antenna to the location of the existing 8-way splitter, mount the new 4-way splitter, move each of the active cables that run to receivers from the 8-way splitter - over to outputs of the new 4-way splitter. The input of the 4-way splitter would of course be fed by the new antenna. That should leave the cable modem connection unchanged. You may want to disconnect the cable that runs between the 2-way and 8-way splitters, and place a terminator on the unused port of the 2-way splitter.
I recommend grounding the new antenna.
http://forum.tvfool.com/showthread.php?t=901