You want to show that the means for four sets of data are different. Use ANOVA analysis to check if the sample means are different enough to say that the population means are different.

If the F-ratio is over your critical F ratio then there is at least one season that has a different mean to the others. This wont tell you which one is different.

Note that if your F-ratio is below your critical value this does not mean that the means are the same, it just means that you are unsure if they are different or not.