depending upon the requirements, just activate what you need - there's no advantage to connecting unused ports to switches. This would just be an unnecessary expense in terms of extra switches, power and cooling. Add new switches as you need them.
Getting a future projection on how the site will expand is one question, getting an accurate response is a totally different matter. ;-)
Pre-cabling all 4 is usually a good move since the cost to re-cable after the fact will probably be double.
Can't assist on the Telephony query other than to say, it may be wise to use separate VLANs/subnets for voice.