I'm looking for a reasonably priced house or hotel rooms in Manuel Antonio for one couple and 3 singles from Dec 28 thru Jan 5th.
I need 4 bedrooms rooms total.
Any suggestions?

Check out Mono Azul: http://www.hotelmonoazul.com/. I've stayed in rooms of all different prices in MA (doing my "research) and I like this hotel for it's reasonably priced rooms & friendly staff! :-)
Check VRBO, or Flip Key.
But at this late date, if its beach you want, I suggest looking in a place that is not the most touristy impacted area of the country, like Playa Samara, Tamarindo or Mal Pais/Santa Teresa area...as the beach at MA really is crowded and small in comparison to above suggestions, and the lodging is all above the beach, you have to walk on a road to the beach, and back up the road...ugghh
Really is going to be hard to find anything at any beach...Samara ...Tamarindo Mal Pais Santa Tereza,,,incredible busy...maybe harder to find a single nice affordable room available...at any of those beaches...
During the last week of december...thousands of costaricasn and nice visitors like you looking for nice affordable hotel rooms...
The best is to drive to Manuel Antonio and shop around...is going to be hard...but like anything in life it is possible!
Merry xmas!
roadadvisor