We are considering Buenos Aires for Christmas and would like to know if anyone has experience or recommendations for renting a 3 or 4 BR house/condo? Suggestions for a very nice area to splurge on?

www.apartmentsba.com
www.buenosaireshabitat.com
www.argentinahomes.com/properties

I have only rented 2 bedroom apartments here, homes/condos that big are not as common in rentals, I believe. But here are 3 rental agencies that get very good reviews over all .
You will probably end up looking in the neighborhoods of Palermo or Recoleta.
Happy hunting

For something a bit different you may wish to check out the Puerto Madero area down by the docks (apparently modeled on Londons dockland. We viewed a few apartments down there with a view to buying and they are both luxurious spectacular!! Many have fantastic views of the city or the river (or both) some have indoor and outdoor pools, health spas etc. (Damn, writinag about them makes me wish I had bought taken the plunge an bought one!).
It is a little out of the usual areas for renting but has some great restaurants and bars and is buzzing at night. It is also very safe as it is patrolled by teh navy rather than normal police.

crellston, isn't it amazing? Puerto Madero.
We were in BA in 2007 and it wasn't half as built up as it is today .. even now, when we just go over for a walk or lunch, I am amazed at the changes ..
It is so clean too and all those unbroken sidewalks lol...

I'd search in Palermo Viejo...there are some houses that you can rent which are raelly big- you might also want to try VRBO.com (Vacation Rentals By Owner). They are pretty pricey but when you see the properties they are impressive!