I'm no expert, but I grow them every year. I hope they have plenty of room to run. The mound is a good place to start, but they will need 10 times the space. Watermelons need lots of food. 1 cup of 4 4 4 is really not that much and I would definitely apply the whole gallon to the mound after prewatering. Nothing wrong with applying the fertilizer directly following the instructions on the package. You could probably sctatch in a cup or 2 right away. I'd also get a little stronger fertilizer. Just my .02

First of all, you have too many plants for that little space.
Secondly, you should have fertilized the mound with some 10-10-10, before you planted the seeds. Third, you should feed a high nitrogen fertilizer, when they start to run. This can be a liquid application. No other fertilizer is necessary, but melons need lots of water.

Thanks for the help. They do have a lot of room to spread so I'm not worried about that. I went with the 4-4-4 because it was the only organic fertilizer I could find. I assumed I could times the application by 2.5 to get the 10-10-10 ratio.

It isn't the room for the plants to spread that is the concern. The concern is the lack of room for the roots to spread and compete with one another.

Impossible to advise you. You need a soil test to determine if you need to fertilize.

I have a soil test done annually, and get fertilizer recommendations. If my soil has adequate N-P-K when I plant, I am advised to not add (side dress) fertilizer during the summer, as watermelon does not need it. (Not a big user of N, and P & K will stay in place in your soil through the growing season.)

Spend the $15 for a soil test from your state Ag. college for a test and fertilizer recommendations and forget asking for opinions from people who have no idea where you even live.

I tell them what I am growing, and I am advised when and how much to fertilize onions, cucumbers, tomatoes, corn, beans, peppers, muskmelons, asparagus, and not to fertilize lettuce, carrots, radishes, and watermelons beyond the beginning of season basic fertilizer added to the whole garden.

The cost of the test will save you money you will otherwise spend on unneeded fertilizer.

Yes you can double the dose to get the 10 10 10. If they look like they are growing well your ok. Mine are really starting to run, but I grow in containers and the fertilizer requiremants are different. Growing in the ground requires less fertilizer.