The ISP is sending four tagged public IPs (VLAN 30) through one WAN cable. Currently, this cable is connected to a switch that recognizes the tag and distributes it appropriately. Naturally, a router needs to be implemented between the ISP's router and the switch to obtain private IPs. I have a Cisco 881 on hand. Is there a way to forward the private IPs with the tag (VLAN 30) to the switch while balancing the bandwidth between the four public IP´s to ensure maximum speed? I have approximately 100 end users. I am pretty new to networking.
Is there a way to forward the private IPs with the tag (VLAN 30) to the switch
You can use an access port on the switch with VLAN 30 untagged, or a trunk port on the switch with a VLAN subinterface on the router. Either works.
while balancing the bandwidth between the four public IP´s to ensure maximum speed?
Bandwidth isn't about IP addresses, it's about link speed. Since there's just a single link ("one WAN cable"), there's nothing to balance.
You can bind all public IP addresses on the router itself and then forward specific ports (destination NAT) to private hosts as required. Alternatively, you could connect up to four devices to VLAN 30 on the switch and use them in parallel - of course, all devices share the bandwidth of your WAN link.