Hi,
There is no options for that by default, but via a plugin or potentially a third component that could be done.
If you create a new group, like "discount_group", add the price of USD 515 to the desired product and restrict the price access to the group previously created.
Then, when the customer register, he can be added to a group, so you have to do some checks to see in which group he is added, and use code to check how many users are in this group. If there is less than 4 users, so do nothing, but if there is 4 users, add all of them to the "discount_group", else if there is already more than 4 users, so just add the current user to the discount_group.
You can potentially create a plugin based on the function onBeforeUserCreate(&$element,&$do), onBeforeUserUpdate(&$element,&$do).
Hope this will help you.