I do full no-sparges on ALL of my beers and I rarely go over 3 qts/# W/G ratio. Your main issue with going over that amount is you are not only thinning out the mash but also thinning out your enzymes which can make them work more slowly simply because they are now floating around in more water rather than working on conversion. This can affect your overall efficiency and therefore your OG.
I like Bob's idea with mashing at a more normal ratio for your saccharification rest and then maybe add your additional water to hit a mash-out temp of 170F or so. At that point, your conversion will be complete with a normal W/G ratio and adding your additional water will bring up your mash temps prior to running off into the kettle. Just be aware that you don't go too much higher than low 170'sF which could lead to extracting too many tannins leading to astringency especially if your pH gets too high.