We've been trying to conceive for almost four months but i can't get pregnant, is that a sign of infertility?"

No: Infertility is commonly defined (for someone under 35 years old) as the inability to conceive for one year. You do not meet that definition. Keep trying. Good luck.

Not necessarily: Depending on your age, this is not a long time to be trying. Typically in women under 35 we do not recommend starting an infertility workup until you have been trying for one year.