Hi Norman,
The Weather Channel shows the average January temp for Orlando as a high of 72 and a low of 50. Being from New England, my kids would consider that to be warm and would want to swim. The pools are all heated year-round to a temperature of 82 degrees. Since the weather can be so unpredictable, you'll pretty much need to wait and see how warm it is when you are there.
Typhoon Lagoon, which is a great Water Park, will be your only option. This is because
Blizzard Beach is currently scheduled to be closed for refurbishment from January 16 to March 27, 2011.
Honestly though, if you have 4 full days, I'd recommend that you spend them in each of the main Theme Parks. The price difference between a 3 day Magic Your Way ticket and a 4 day one is $8.00 total per person! A day at Typhoon Lagoon will cost you almost $50 per person extra. I'm not sure which Disney Resort you're staying in, but all of them have wonderful themed pools which you could enjoy in the evenings. I would have a very hard time choosing which Park to recommend that you skip, due to the ages of your kids. Each Park has something that you won't want to miss. Magic Kingdom is a must, as is Animal Kingdom. After that, with a 4 year old, I consider it a toss-up between Epcot and Hollywood Studios (HS). While HS is my older kids favorite Park, but my younger ones don't care for it yet. With the exception of one or two rides, the only thing they are able to do (because of height restrictions) is to watch the shows. I personally find there is much more for them to enjoy in Epcot. That being said, HS has several rides that are likely to be your 10 year old's absolute favorite of the whole trip. This is why I think you should visit each of the 4 main Parks and save the Water Parks for a future Summer trip.
Have a wonderful vacation,
Kaylene