The syn 4 will give each of the of the pro 60s 125watts, the pro 60s are rated at 150watts rms so they will be slightly underpowered by the syn 4. I honestly doubt you would notice much of a difference in volume between having 125 and 150 watts to the pro 60s.
If you never plan on upgrading the pro60s check out the arc audio ks300.2 it would give you 175 watts to each of your 60s and cost you just $25 more brand new.
http://store.arcaudio.com/products/p....0.0.0.0?pp=8&
If you think you might upgrade to pro 80's or rev8/10s someday you might wanna look into a 2-channel amp like the arc audio ks600.2, but you would have to be careful and keep the gains way down on the amp while you are running the pro 60s.
http://store.arcaudio.com/products/p....0.0.0.0?pp=8&