I think you are limited to 3 parties (3 phone numbers) in a conference call on the Verizon Wireless network.
I'm most definitely NOT a VZW employee. If a post answered your question, please mark it as the answer.
Your best bet would be to use a conference call set up like http://www.freeconference.com/ or an internet meeting online like http://www.webex.com/ Both have free versions (though the call in numbers for the conference call and audio on the webex meeting are not toll free and would use plan minutes for most people) and I have used webex several times for "meetings" with clients in Wisconsin (and I am in California). Sure beats paying for plane fare!