In Spain I learned aquí, allí, allá, but in Costa Rica it all seems to become acá. Am I not remembering the Castellano correctly or does this differentiation not exist in Latin America?
"Aquí" and "Acá" have the same meaning. The same for "Allí" and "Allá".
We use "Aquí/Acá" when you're talking about a position near to you, or maybe your own position. In the other hand, "Allí/Allá" is for pointing a position far from you.
Aquí/Acá fue donde lo vi por última vez.
Esta silla de aquí/acá es de hace 5 años.
Vamos a comer allá/allí
Ese de allá/allí es a quien buscas
I was taught - by a Colombian - that aquí is more proximal than acá. That is, if something is in the general vicinity, but not at arm's length, one would use acá. If it was easily reachable, one would use aquí. (Exactly where the boundary lies is a bit fuzzy).
Is this the case in Colombia, but not elsewhere?
As the RAE definitions compared in this other answer show, both words have the same meaning. However, from the questions and comments it is clear that in some regions one is preferred over the other.
By using Google trends (thanks @ukemi for this find), which shows what people are searching for, we can notice that:
- aquí is overwhelmingly preferred in Spain
- aquí and acá have mixed usage in hispanoamerica, with aquí being more popular overall but acá getting more common the further south you go
- Argentina is the only country where acá is more common than aquí
Since the language evolves here is the result of Google trends as of May/2019
In the particular case of Argentina, again according to Google trends the preference for acá has not been always the same and around 2012 aquí was more common in searches.