MAS 4.0 and UCA 5.0 are both available for download/upgrade from Mitel.
Included in UCA 5.0 are the new clients. There is a new Desktop Client, Web Client Portal, and iOS/Android Client [available on their respective AppStore/Google Play].
I'd have to review the MAS 4.0 Engineering guidelines to provide such specific details/recommendations. I'd also have to know the configuration of each of your MAS servers [i.e. the blades on each, and if they are LAN only, DMZ, Network Edge, etc.]
Do access UCA from outside of your office?