Q.Melon Trellis
I have a 4’x 7′ garden, which I know isn’t much room, but I wanted to grow watermelons, honeydew melons, cantaloupe, and cucumber. I am going to grow them on a trellis, which needs enough strength to support these types of plants. I want a style of trellis that I can easily access the fruits without having to crawl or reach under and squeeze between the frame of the structure. I have looked at A-Frame structures, Arches, Pyramids, and a few other types of structures, but I wanted to know which one would work best as far as accessibility given the space that I have.

Certainly the vegetable you are growing will be the biggest deciding factor on the trellis system that you use.
These links will help you.
https://www.gardeningknowhow.com/edible/vegetables/vgen/growing-a-vertical-vegetable-garden.htm
https://www.gardeningknowhow.com/edible/vegetables/vgen/using-teepee-structures.htm
https://www.gardeningknowhow.com/edible/vegetables/cucumber/planting-cucumber-fence.htm
https://www.gardeningknowhow.com/edible/fruits/watermelon/watermelon-vine-support.htm
https://extension.missouri.edu/p/M173
http://www.extension.umn.edu/garden/yard-garden/vegetables/trellises-and-cages/