What do i need to provide a VPN link btn a main Office and 3 remote Office

I intend to setup a VPN link between A main Office and three(3) remote offices. I have an IOS image of 12.4 (1c) on router 1841 at the main Office which i understand needs to be upgraded in order to achieve my goal.

My question is:
In buying the Advanced security, do i need to buy 4 sets for each office. What options are available