You can't say "I can swim since I was four", you must say "I've been able to swim since I was four". Can we use for ability in the present, what we can do now "I can drive". Could we use for ability in the past, what we could do in the past "I couldn't drive when I was ten". When you say "since I was four", you are talking about a period of time (from when you were four until now). This is the present perfect tense. For the present perfect, future, infinitive and conditional forms you use 'be able to'. "I'll be able to pass the exam soon" (future) You can use 'be able to' for the present simple and past simple, for example "I'm able to drive" or "I wasn't able to drive when I was ten", but it is more formal.

The issue here is grammar and tense use rather than meaning. "I can swim since I was four" is not correct. We can't say this. The time period 'since ........' requires a present perfect tense, because it focuses on something which began in the past and and is still true today. When we use time markers such as 'since I was four' or 'since 2012', we are focusing on the whole period from then until now, and we have to use a present perfect - NOT a present tense. For example, you could sat 'I've spoken French since I was four' or 'I've worn glasses since I was four' or 'I've known him since I was four'. These sentences use the verb 'have' plus the past participle. In common with all modal verbs, the verb 'can' has no past participle, so it cannot be used in perfect tenses. Instead, we use the synonym 'be able': I can swim. = present tense I've been able to swim since I was four = present perfect to talk about an ability which you have had since you were a child I hope that makes sense.

I hope this helps!