Your first Silk Road (assuming 3 starting Estates) is worth 2 VP. Your fifth Silk Road is worth 4 VP on its own and adds another 8 VP by increasing the value of the first four Silk Roads.
No it's not. Your first Silk Road is worth 1 VP, yielding 4VP in total with your Estates. If you don't buy any other Victory cards, your fifth Silk Road is also worth 1 VP. With Silk Road in play, your Victory point total would be s(s+v)/4 - ((s+v)mod4)/4 + V, where s is your number of silk roads, v is your number of other victory cards, and V is your victory point total otherwise. Silk Road is linear - just multivariable. If you had all the wacky victory cards in play (gardens, duke, vineyard, fairgrounds, silk road), the sum equation would be:
V = s(s+dk+vy+f+g+dch+v)/4-((s+dk+vy+f+g+v)mod4)/4+g(s+dk+vy+f+g+dch+v+a-av+o)/10-((s+dk+vy+f+g+dch+v+a-av+ o)mod10)/10+dk*dch+vy*a/3-((vy*a)mod3)/3+2f*N/5-((2f*N)mod5)/5+C
where V is your total VP, s = silk roads, dk = dukes, vy = vineyards, f = fairgrounds, g = gardens, dch = duchies, v = other VP cards, a = action cards, av = action-victory cards, o = other cards, N = number of different cards in deck, C = conventional victory points, through chips or non-counting VP cards.