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Most corn you see for sale in your local big box store will by hybrid. Most seeds sold commercially (corn or otherwise) are hybrid seeds, as hybrids are bred to be more disease resistant and do not reproduce true (which means the seed seller has a guaranteed repeat customer).

In today's world, if the seed you buy is not marked "heirloom" it is a hybrid seed. Here is more information on hybrid seeds: https://www.gardeningknowhow.com/garden-how-to/seeds/non-hybrid-seeds-vs-hybrid-seeds.htm

The ear of corn is ready to pick from the stalk about 20 days after you see signs of the first silk growing. Here is more information: https://www.gardeningknowhow.com/edible/vegetables/corn/grow-sweet-corn.htm

If it's tasseling at 4 1/2 feet tall, it's probably fine. It will grow at least another 2-3 feet as the tassel emerges and the stalk elongates. Early tasseling is often brought on by extremes in weather. Just make sure to keep the corn watered well.

This is actually more common than you would think and is normally weather related. Corn tends to tassel early if the plants are hit with a cold snap early in their development, usually from planting too early. It can also happen when the weather becomes way too hot. There is really little you can do, but the corn will still continue to grow.

They may not be getting enough water. Corn has very shallow roots and needs lots of water during its active growth, especially in high temps. Typically, the garden needs about an inch or so of water each week. It is a good idea to water deeply (for about 20 min. or so) every other day or two, depending on the weather. Once temps reach 85 F and up, daily watering is often necessary.

Nearly any type can grow in warm temps; however, since corn has very shallow roots, it needs lots of water during its active growth, especially in temps as high as you are experiencing. Typically, the garden needs about an inch or so of water each week. It is a good idea to water deeply (for about 20 min. or so) every other day or two, depending on the weather. Once temps reach 85 F and up, daily watering is often necessary.