None of my kids (nor I at that age) ever had an Easter, or a spring break party. (Even in a Christian preschool. Just not a "party" type of holiday, I guess.) Halloween, Winter Holiday, Valentine's and End of Year are it. But you could do something fun with summery stuff in a sandbox for an end of year party!
Four kids in a class? Wow! Now I'm really curious. Where is this? Is it a special needs class of some sort? Recent budget cuts have increased our elementary school class sizes to 27-30 on average, but even back when we had enough teachers, most classes were 22-24 and the smallest I ever saw was 15 in an afternoon kindergarten class (no one wanted afternoon K ).