Is additional memory required to use the Boost license on a ISR 4331?
We are looking to add the Boost license to an ISR 4331.
I heard that additional memory is required beyond the default 4 Gig that comes with the 4331.
But I cannot find that stated in any Release Notes or other documents.
Is anyone using the Boost license on a 4331 with 4 Gigs of memory?
No additional memory needed. You can think about it as if Cisco implemented a "QoS" on the router in terms of performance and with performance license you are disabling the "QoS. ". But the router is already made to support the maximum it can get.
As others have already noted, the answer should be no, as all the boost license does is remove the software imposed shaping for all aggregate traffic.
As they also noted, because of the increase throughput, if you do "more" with your traffic than just forward it, it's possible an increased level of throughput might increase memory requirements. (Again, much would depend on what you're doing. Two possible simple examples, more traffic flowing through the router might create more ARP entries in the same time period. Likewise, more traffic through the router might increase the amount of netflow entries in the same time period.)
However, unless you're doing something really unusual, I wouldn't expect there to be any major increase of memory usage, unless, you concurrently, decide to enable some feature you haven't been using.
A good example of memory impact, due to current throughput, before you upgrade your license, might be: compare memory usage at the least busy time of day vs. the most busy time of day.
BTW, as an aside, also keep in mind a boost license uncaps the performance limit, unfortunately there's no guarantee how much of a boost you'll actually obtain. It might range from very much to very little. This because, difference feature usages differ in the amount of CPU they use.
Possibly, though, as a good clue, if you examine your CPU usage when its the busiest, and the volume of traffic flowing through your routers at that time, that will give a rough guide to how much reserve CPU is availalble.