I know Google hits aren't necessarily reliable, but I nonetheless searched for the following two forms:
The numbers are much further apart if I search for the -い form:
As I understand it, 暖かった is considered prescriptively incorrect because it looks like the past form of the non-existent adjective ×あたたい. However, if this form has really become more common than 暖かかった, then I suppose it's an acceptable form. Unfortunately, I can't base that conclusion on Google hits alone, so I decided to ask here. My guess is that it's easier to say あたたかった than あたたかかった, so one of the かs gets dropped.
If you can drop the か, I'm curious whether you can contract あたた to あった at the same time, as in option four below:
Are all four of these forms in common usage? Are some of them considered non-standard or colloquial?