You could have a look at hotels such as the Tubkaak and the Amari Vogue in Krabi. They will be expensive that time of year as it is peak season. They are also in a very quiet location with few, if any local restaurants around, but are on a good beach with easy access to outlying islands such as Koh Hong. Or if you want more life you could head to Ao Nang. The beach there is not great, its mainly used as a staging post for day trips to outlying islands and Railey beach which is a stunning location. Ao Nang itself is a lively family friendly resort with plenty of bars and restaurants. You could also take a look at Phi-Phi island, although this would involve a transfer by boat from either Krabi or Phuket and you only have four days hols so you don't want to waste too much of it travelling. The same applies to the island of Koh Lanta.
Getting to Krabi is easy as there is an airport, from there its a half hour transfer to Ao Nang, or transfer from Phuket which takes a couple of hours.
In previous years (prior to the economic downturn) many hotels will be booked out this time of year so best to book up soon.
If you give us some idea of budget and the type of holiday you are looking for someone should be able to help.