I also don't think WAN accelerators could do a great benefit to encrypted and latency relevant workloads.
Did you try optimizing PCoIP at the Agent with the ADMX Files?
Configure the PCoIP session bandwidth floor
Specifies a lower limit, in kilobits per second, for the bandwidth that is reserved by the PCoIP session.
This setting configures the minimum expected bandwidth transmission rate for the endpoint. When you use this setting to reserve bandwidth for an endpoint, the user does not have to wait for bandwidth to become available, which improves session responsiveness.
Make sure that you do not over-subscribe the total reserved bandwidth for all endpoints. Make sure that the sum of bandwidth floors for all connections in your configuration does not exceed the network capability.
The default value is 0, which means that no minimum bandwidth is reserved. When this setting is disabled or not configured, no minimum bandwidth is reserved.
This setting applies to Horizon Agent and the client, but the setting only affects the endpoint on which it is configured.
When this setting is modified during an active PCoIP session, the change takes effect immediately.
80ms is quite high, i have sub 30ms and it is working fine. Both Blast and PCoIP.
ATM i prefer Blast in my Home Office from a fat client.
A WAN accelerator will not help. Your best bet is to optimize the PCoIP settings by reducing the image quality and capping frame rate or setting a bandwidth limit. The reality is that what you are asking for with 4 screens depending on how much and how often the screen is "redrawing" can take up a lot of bandwidth. I'd also make sure your environment is current.