Iptables – Linux equivalent of pfsync+carp for failover firewall/routing

Presumably your 4 Linux servers have at least two nics? Do they actually need the combined bandwidth of both nics to handle their normal load? If not then some of the bonding modes provide for redundancy without using etherchannel.

Anway, see the bonding document it provides lots of several example HA situations depending on what you need, and what your network hardware is capable of.

Excerpt from bonding.txt

11.2 High Availability in a Multiple Switch Topology
----------------------------------------------------
...

    Below is a sample network, configured to maximize the
availability of the network:

                |                                     |
                |port3                           port3|
          +-----+----+                          +-----+----+
          |          |port2       ISL      port2|          |
          | switch A +--------------------------+ switch B |
          |          |                          |          |
          +-----+----+                          +-----++---+
                |port1                           port1|
                |             +-------+               |
                +-------------+ host1 +---------------+
                         eth0 +-------+ eth1

    In this configuration, there is a link between the two
switches (ISL, or inter switch link), and multiple ports connecting to
the outside world ("port3" on each switch).  There is no technical
reason that this could not be extended to a third switch.

11.2.1 HA Bonding Mode Selection for Multiple Switch Topology
-------------------------------------------------------------

    In a topology such as the example above, the active-backup and
broadcast modes are the only useful bonding modes when optimizing for
availability; the other modes require all links to terminate on the
same peer for them to behave rationally.

active-backup: This is generally the preferred mode, particularly if
    the switches have an ISL and play together well.  If the
    network configuration is such that one switch is specifically
    a backup switch (e.g., has lower capacity, higher cost, etc),
    then the primary option can be used to insure that the
    preferred link is always used when it is available.

broadcast: This mode is really a special purpose mode, and is suitable
    only for very specific needs.  For example, if the two
    switches are not connected (no ISL), and the networks beyond
    them are totally independent.  In this case, if it is
    necessary for some specific one-way traffic to reach both
    independent networks, then the broadcast mode may be suitable.

This is a little more complex since I'm assuming that your routers can't export their routes in a meaningful way. If you feel like digging into it, remember that when you send a packet locally the IP address is still the far-end host and the MAC address (on Ethernet, anyway) is based on the hosts route table and arp cache.

If you knock your software down a level and use the kernel's packet calls to form your message, you can write that IP packet with the MAC address of your choice. The packet will then use the router of your choice as the first hop. Use that to ping and monitor responses and you'll be able to test connectivity without changing system settings.

Of course, you'll need some elevated permissions in your program to accomplish this.

Otherwise, forced single-address routing table entries suggested by Evan will allow you to stay out of writing things that call on the kernel. For any method where the router's aren't complicit in telling you, you need to be in the DMZ otherwise you can't choose where your packets go unless you somehow end up with source routing enabled.