Going to be staying in Madrid for a week with total of 4 adults. Want to stay near Puerta del Sol. Suggestions of rental apartments would be great. Want to stay in safe area to walk around day or night.
You might want to check out the Slow Travel website. They have reveiws of a number of apartments in Madrid and it's a great source of other information about Spain. Friendly Rentals is a popular site that has apartments in Madrid. I also use HomeAway and Owners Direct. You can also just Google 'vacation rentals Madrid' and lots will come up.
I would second the suggestion to check the slowtravel website. We rented the Puerta del Sol 111 apartment through Friendly Rentals last year and were very pleased with the agency and the apartment. However it would not be suitable for 4 adults as the only bathroom is off the one bedroom. The apartment is fine for 2 but would be too small for 4. Given the Spanish are out and about until late at night/early morning, most parts of the city are quite safe to walk in day and night.