The best way to approach this is to place your prefabs in an array. Each prefab is an element of the array, and you access an element using its index. The index is the position of the element within the array. The first element in the array would be array[0], the second element is array[1]. As you've noticed, arrays and lists start with a 0 index, not 1. This is very important to remember.
First, place your prefabs in an array variable. In your state, use a Random Int action; this will select a random integer between a min and max value. The min value, is 0 (first index), and the max value should be the length of your array. Since you have 4 prefabs, the array length is 4. Store the result in a variable. This will be used to select a prefab from the array
TIP: Instead of inserting 4 for max value, create an array length variable and use the Array Length action before the Random Int action, to save the length of the array to the variable. Then use that variable for the max value
Next, use the Array Get action. Place your array variable and use the stored index variable as the index value. This will give you the prefab from that index and store it in the result. Now, use the result as the object to spawn on the Create Object action
This method is better because you don't have to create an event and separate Create Object action for each prefab. But, there is an even better solution with much less work...