Sure there are firewalls with 4 interfaces, but my question is how you intend to address your routing issue? With two internet connections you are talking two default routes and two different ISP assigned IP spaces.

The poor-mans solution is to put your users behind one firewall, your servers behind another (each with NAT/routing appropriate for the ISP they connect to) and cross connect the two firewalls together with a couple of routes to each others networks. Im not suggesting this is the best way, just an option.

The most common way to implement multiple ISPs is to get your own IANA assigned IP space and exchange BGP with your ISPs. That and use of QoS on your traffic if you comingle user browsing and business critical traffic.