Please post the expense ratios and any other fees, otherwise we can't do much. Remember to only use the 401k up to the employer match, then max out your HSA (if available) and IRA before filling this up. Don't over think the percentages. 81/15/4 is overly precise. You will get a lot more diversification from the international stocks and bonds than you will from a mid cap fund, so consider those if you don't address them in your other accounts.
S&P500 index fund: If the expense ratio is higher than the small-cap fund, I would not hold at market weight. I would over weight the small cap fund, or even go all in that.
Vanguard small cap fund: looks to have the lowest expense ratio of the three, but verify. Use this to replace the midcap fund, and possibly part or all of the S&P500 fund depending on expenses.
QMA Mid Cap Index Fund: I didn't see this on a google search, I only found an active mid cap value fund which should be avoided. Honestly Vanguard Small Cap contains so many midcaps that this isn't really needed.