There is no limit on the number of servers, capacity is measured in messages per hour. This is, nominally, 2 million messages per hour. Real world is, in my experience, 1.2-1.5 million max. This is dependent on many factors and we have seen spikes around 2.5 million message per hour that were handled fine.
I don't use web access but the number of messages that 4gb will hold will depend on your message rate. The standard syslog message size is 1k so based on that you can figure out how many messages 4gb would be and what the max rate would be to keep 30 days worth in the DB.
The log forwarder sends message in standard syslog format. It should work with any syslog server. One thing to be aware of is that Windows event logs can be large and well over the max message size for the syslog standard. Kiwi Syslog server handles this well, others may not.