Yeah, I would second the flexibility of a cap 4 PowerDry High Efficiency top -- either hoody or zip neck.
I have hiked down to 11 degrees F with one as a top layer over a lightweight Power Dry baselayer and been comfortable hiking up to 40 degrees with it as the only top layer. They are also awesome for sleeping.
In addition to Capilene 4 and the MEC option above. Marmot sells the same thing called their Thermo Hoody and Thermo 1/2 Zip.
For a 4.4 ounce option, I've been turning to the Patagonia Nine Trails jacket as the first thing I grab hiking when I need a little more. It's a non-hooded, full-zip Houdini, but with stretchy breathable fabric panels across the top 2/3rds of the back, down the sides, and under the arms. This thing is awesome for providing a bit of wind protection, a bit of warmth in the torso, and still being extremely breathable. Stuffs into it's own pocket (small of the back) and takes up virtually no room in the pack. No hood, but otherwise this is a really awesome hiking piece for a lot of conditions. With a ball cap, it would even be suitable for a little snow, drizzle, mist, or mist.
I have a couple of vests, but I don't find myself reaching for them hiking very often. When I need something extra, it's usually because there's a breeze involved and I need the extra on my arms, too.
Most of the fleece vests I have seen are Polartech 200 weight fleece (or equivalent). That's a pretty heavy fleece. Way too hot for me when I'm hiking. Maybe for around camp, but with zero wind and zero moisture protection, I don't think traditional fleece offers a lot of bang for the ounce in the backpack.
REI has a PowerStretch vest that would be lighter (more like PolarTech 100 fleece) that would be a better warmth for hiking, but I doubt that it's anywhere near 5 ounces. It's going to be more like the Patagonia R1 gridded fleece vest, which is 9 ounces compared the Cap 4 zip neck at 6.7 ounces. I can't wear PowerStretch or R1 fleece for strenuous hiking unless it's well below freezing. too hot.