Link to Miercom 4k performance analysis: https://miercom.com/pdf/reports/20150817.pdf
Cisco ISR Performance Overview White Paper attached.
Again, the takeaway for the forgoing, the possible impact of different traffic mixes and/or router configs. Neither, though, addresses the impact of some control plane processing. (Real example: years ago, had 3660 running with a single DS3, which loaded the CPU to about 30% at max DS3 utilization, but when I took full IPv4 Internet tables, the BGP scanner, alone, crushed the CPU.)
I've also mentioned, in Cisco routers the next step above ISRs, some of their special acceleration hardware, such as the PXF, QFP (see flow processor in the link), (the Catalyst 8K uses the 3rd generation of the QFP, and even possibly more "network processor" components, like the Q100 or Q200 - I doubt you'll need the latter, but wanted to show why ASRs and/or 8Ks, can offer much higher performance, using such hardware for the data plane, lessons the impact of forwarding performance using just "ordinary" CPUs).
Now again, am I saying a 4461 wouldn't be ideal? I'm not, because I believe there's yet insufficient information to say for your expected usage. However, as far as I know, the 4461 is NOT dual 10g sustained wire-speed capable under all conditions.
What does Cisco say, regarding a 4461's performance:
It appears to offer about 50% more than a 4451, and the latter is benchmarked in the Miercom report.
In that report, for just IPv4 forwarding (NB: IPv6 throughput is often less than IPv4, another variable), we see the 4451 hits 2 Gbps but with only an 18% CPU load, so it appears, uncapped (boost license), we should be able to achieve about 5x that, or 10g. And if, the 4461 does have an additional 50% performance, we're now up to about 15g, which is not 20g needed for dual 10g.
If best expected throughput of about 15g is acceptable (discounting any other CPU demand for other "things"), for IPv4 alone (possibly less for IPv6), then you should be fine.
Lastly, as you have ASRs in operation now, studying their stats allow about the best way to estimate what 4K ISR, if any, might be a suitable replacement. Again, though, a 4K ISR is not an ASR, so direct performance ratios are not a "given". (BTW, the Cisco White Paper, which has different ISRs, will also often show, performance doesn't "scale" the same way between various models.)
