Watermelons will grow in just about any kind of soil, as long as it's loose. They also need lots of room and 4x8 isn't very much. That might accommodate two plants at best.

I grew watermelons this year in a raised bed in home made compost. I grew Crimsons this year and they were the size of Basketballs. Every few weeks I added 13-13-13 or Miracle grow. Watermelons like a lot of nitrogen.
My homemade compost is manure free, as I dont raise cows and dont know what they are fed.
My compost is 70% browns and 30% greens. I grew 3 this year and they did great. I still have watermelons coming in

As an experiment I grew cucumbers in the same bed, in a tomato cage. I had a bumper crop of cucumbers.

I also did my cantaloupes the same way as the watermelons, except it was 50% compost and 50% soil and had a great yield as well

It has been too cold this year for watermelons. They are easy to grow if it is hot.

Watermelons like loose and rich soil, but I have raised my best perhaps in heavier virgin soil.
This year in central Indiana it has been cooler and ripening was delayed, but they were of large size.