The main choice:
1. Go digital (VOIP).
2. Stay analog (PBX).
The second choice:
1. Choose and install yourself, buy online.
2. Use a local company for advice, installation and support.
I can't see the relation between having 4 lines and you not being able to "control your staff because of communication problem". Could you tell more on that and why you thing a PBX or a VOIP installation might help?
Also note I might delete some spam answers to your question. There's a certain small company like you're looking for spamming these forum lately.
I have a small business. There are ten employees working in my organization. Actually, I?m having 4 separate telephone lines which are quite costly for me and also I?m unable to control my staff because of communication problem. So I am planning to install a PBX system that I can save my money and time. Does anyone know a good source where can I buy?