Cisco 4K ISRs don't support "connections" they license performance based on bandwidth consumption rates (which varies based on actual PPS and packet sizes, i.e. for the same "bandwidth" consumption, PPS might be high with small packets or PPS low with large packets - also this is assuming you're not running with a "boost" license [which you note your are] - the latter allowing the ISR to forward whatever it's physically capable of doing based on packet rate, packet size, packet kind/content and your router configuration).
For a 4321 with boost license, Cisco documents the 4321 capable of up to 1.5 Gbps (see table 4 in: 4K ISR data sheet) but don't count on achieving that rate all, or perhaps even most, of the time. (On prior gen ISRs, I found you wanted to divide the max rate by at least 4 for a useful actual max rate. In this case, the ISR 4321 might support 300 to 400 Mbps, using its boost license. It's also possible for your traffic, higher [that 300 to 400 Mbps] might be possible too. Again, though, I wouldn't count on much beyond the performance license level.
Study the 4321 performance tests in: Miercom 4K performance analysis, page 11. You might be able to "scale" expected performance, for the various traffic kinds, and feature[s] used, based on CPU load at the base and performance rate. (Do keep in mind, differences aren't always pure linear, as might be seen in the base and performance rate tests and, normally, you want some "reserve" CPU, the latter Cisco often recommending a 25% reserve. [I.e. try to avoid exceeding 75% CPU load, for your general average.])
