I think 4 days in the Cusco area is about right, I don't much care for the city itself but the Sacred Valley is nice.
I second Pisac, probably cooler when the market is not in full swing as they bus tourists in from Cusco. If I remember rightly you could do the market in the morning then the ruins in the afternoon and that should avoid the crowds.
The ruins you probably only want to do one way, so there are profiteering taxi drivers at the end to return you to Pisac. Expensive but unless you are a weathered trekker it's your best option.
Also near Pisac, we borrowed someone's scooter and went to Huanca, a creepy place worth a look, everything is centred around the church of El Seņor de Huanca.
A dining tip, not that all Peruvian food isn't amazing but the Cuchara de Palo at The Pisac Inn in the Plaza is brilliant value for money and their Alpaca brochetas with chaufa de quinoa was delicious. They also have a nice fire so you can put your feet up and dry your socks after las ruinas.
http://www.pisacinn.com/restaurant.htm
You will see houses and alleyways with a basket or a carrier bag on a stick. Baskets = good, well worth sticking your nose in there because they make delicious Andean bread in big open log ovens. Carrier bag = not so good, they make hot chicha and serve it out of a bucket. Depends how involved you want to get with the culture but it's a mistake I won't make again
burp.