You have 4 hens and one tom your question is why the hen setting on their eggs this first season for does that make a difference?

A hen may sit on her eggs in her first season for various reasons. It is not uncommon for hens to start laying eggs and becoming broody (wanting to incubate and hatch eggs) during their first laying season. Here are a few factors that could influence a hen's behavior:

1. Natural Behavior: Setting on eggs is a natural behavior for hens. Even if they are not fertilized, female hens may instinctually exhibit broodiness and show a desire to incubate eggs.

2. Breed: Some breeds of chickens, such as bantams or heritage breeds, are known for being broody and have a strong maternal instinct to incubate eggs.

3. Environmental Triggers: Certain environmental factors, such as longer days and an increase in daylight hours during the spring, can stimulate the hen's reproductive hormones and trigger broody behavior.

4. Number of Eggs: The presence of a sufficient number of eggs in the nest may also encourage a hen to sit on them and attempt to hatch them.

It's worth noting that the first laying season is not the only time a hen may go broody. Hens can display this behavior multiple times throughout their laying years. However, the frequency of broodiness can vary among individual hens and may be influenced by various factors, including breed, genetics, and environmental cues.