Summary - A drive up on the west end of the Toutle range. Best done in combination with the three summits at the other end of the range, 4562, Castle, and Spud or with Elk Rock (requires a Weyerhauser rec permit) in the next range north. A mast is recommended, two meters is questionable, ATT service is excellent. Drive time from Portland is about ninety minutes (See my 4562 trip report.)
TL;DR - Signal Peak is an easy drive up but otherwise doesn't have much to recommend it unless you are already in the vicinity. The summit has two large communications towers surrounded by dense brush and is without views. Bring a mast unless you want to beat through the brush to the taller trees on the edge of the clearing and don't count on two meters for contacts. Turn off Road 4000 at 46° 17.22', 122° 34.07', the next right takes you to the summit.
On top you will find two communications facilities in a broad brushy clearing, parking, and little else. A mast will be handy if you don't want to beat your way through the brush to the trees on the edge of the clearing and two meter contacts are unlikely. ATT service is, unsurprisingly, quite good, as is APRS. I didn't notice any appreciable RF interference.
Signal Peak is an easy four pointer and best combined with the other Toutle Range summits or, if you have a Weyerhauser rec permit, Elk Rock (W7W/LC-060). Otherwise it doesn't have much to recommend it, even if you live in Toutle, Washington.