Why does the end of my zucchini turn black and gets mushy when it gets to be approx. 4" in length?
Your zucchini has blossom end rot. It is caused by a lack of calcium. This is caused by uneven moisture or high ammonia fertilizer or salty soil.
Add some calcium, like crushed egg shells and keep the soil moisture even. If the water sits for a long time, you have poor drainage, which will also cause blossom end rot.