I want to upgrade to the Bronze level but only for the months of July-October. Can I choose just four months and charge to my credit card?

We offer a monthly option but it would need to be cancelled when you no longer want the subscription.  Simply downgrade your account back to a Free account when you no longer need the upgraded limits:

https://www.jotform.com/help/341-How-to-Downgrade-Subscription-Back-to-Free-Account