Down Payment Advice

I am saving for my first home, and I currently have about 81k in VTSAX. I plan to use most of it for a down payment in 3 to 4 years, around 120k, and I will be adding more as I go. Since the market feels highly valued, should I move some money into VBTLX to lower risk, since I will need the cash sooner? Or should I just keep it in VTSAX and not worry about timing the market? I am open to other strategies, too. I expect to save about 70k a year, so I should reach my goal either way, which makes me wonder if it makes sense to just stay more aggressive.