If you have a hydrometer, you can take a reading and see if it is somewhere close to the final gravity target for your recipe. (It probably is). If so, yes, everything is fine. (If not, your fermentation may be stuck.)
What kind of yeast did you use? Did you use a starter? I find that, with a yeast starter, it's not all that unusual for airlock activity to be quite minimal by the fourth day.