Thanks for your feedback!
You're correct that our main desktop product is only available for PC. Did you know we do offer a fully web-based solution, Act! Premium Cloud? This works great on PC and Mac, as well as keeping your data online on our hosted cloud servers.
It sounds like your business could make good use of the features available via Act! Cloud - might be worth checking it out at act.com - we do offer a free trial too.
We'll definitely be taking your feedback on board for future releases - especially what you've said around email syncing. We've got some major improvements planned in this area in the next release (20.1).
Lastly, there should be no limit to how many contacts can be assigned to a company - if you're having issues adding more than 4, I'd definitely recommend contacting customer support, who'll be happy to help.
If there's anything more we can do to help, please let us know!
Act! Social Media Team