4 months is fine, though I'm not sure the effect of the amount of trub you have in there on the yeast's viability. I've read that it can have a negative effect, but I really can't speak from experience. I've used washed yeast that was sitting in the fridge for over 7 months in a starter, and despite a longer lag time before showing signs of life on the stir plate, it performed just fine given adequate time. I did a 500mL starter stepped up to a 1.25L starter, just to make sure I had enough viable cells.
But, if that's just a common strain such as S-05 and it will only cost about $2 to use a new packet I would just go ahead and jump for fresh yeast.