First, I don't see it as "trying to time the market" in this case -- you're not selling now to buy back in later, you are transitioning from a pure growth period to one in which you will need to draw some income. The reality is, there *will* be a market downturn -- probably several -- over the extended period of your planned retirement, so of course you want an asset allocation that will allow you to weather that. So to me, it makes perfect sense when you are a few years out to start to think about the long-term repositioning of your assets. Although, like PizzaSteve says, you are in a somewhat different position, because you will get a big influx of cash from selling the house, so you could wait and use that for your cash stash.
The thing to keep in mind here is that the 4% rule generally works if you are getting market-based returns, so if you choose to keep a significant portion of your assets in cash-equivalents, that will bring down the overall return, and you need to take that into consideration in your planning. FWIW, I also plan to keep several years' expenses in non-stock assets (e.g., CD ladder, bond ladder). But I am not counting that in my base assets that I count towards my 4%. Yes, it is overly conservative, but I am in a field in which it would be much, much harder to re-enter once I leave, so I consider it retirement insurance.
