As I don't have inside Cisco information, can only but guess why 4 ms Tc is being used. It more accurately matches a physical interface of the same bandwidth, and better supports the service needs of traffic like VoIP.
As to your drops degrading performance, that's certainly possibly, but your overall drop percentage is only about 0.04%. Traditional drop percentages up to 1% are considered acceptable, but as that's an overall cumulative stat, we don't know if you might be having micro bursts.
Additionally, if this is a subinterface, other traffic on the port could be causing transient issues.
Basically, insufficient information to say what's the cause of the "degradation", which also isn't clearly defined.
Other causes can be due to your environment. For example, if you're shaping to match some provider CIR, such often mimics bandwidth of the physical interface, but traditional/older Cisco shapers only counted L3, so to keep from overrunning a provider's CIR, you used to need to shape about 15% slower.
