From your original post, you didn't note what kind of link was your 2 Mbps WAN link. If it's a serial E1, not quite sure how you saw 2.2 Mbps. If the link is also a MetroE with a handoff of 10 or 100 Ethernet, which is what I assume the 4 Mbps MetroE upgrade would be, then the 2.2 Mbps probably measures a burst which may, or may not, have been dropped by your WAN provider if the rate was considered out of contract. Much depends on how the provider enforces bandwidth.
Whether a 4 Mbps would seem "faster" to your users; much depends on the nature of the traffic that's crossing the link and the applications being used. Bulk transfers, something as simple as opening a large file, like some slideshow file, will probably take less time. Interactive access, which is often dependent on latency, might see almost no improvement unless the link has been heavily congested and the additional bandwidth reduces its queuing latency.
If there's competition between different flows for bandwidth, then a QoS configuration that minimizes bandwidth hogs from impacting interactive flows, could offer even better user perceived performance than a bandwidth upgrade (it also can be used with additional bandwidth for further improvement).
With regard to your question of making Lotus Notes top priority; normally only real time applications are given top priority. "Mission critical" applications are provided sufficient bandwidth to minimize their queuing delay.
Actual examples are very dependent on the platform and often also on the software version running on the platform.
For your 4 Mbps MetroE upgrade, if supported by your platform, I would first recommend shaping at that rate since providers often use a combination of FIFO and policing. Router shapers appear to support WFQ. If the performance of your mission critical application of Lotus Notes isn't acceptable, I would then suggest increasing its IP Precedence up so that the WFQ shaper provides in more bandwidth. If this still is insufficient, then, if supported, you might want to migrate to CBWFQ configuration providing your Lotus Notes traffic a minimum guarantee of bandwidth.
