Your last item is often situational.

FON - Keep if the enemy AP champs have scaled very well into late game and are doing loads of damage to the point where you feel you need the extra MR to tank a little better

Aegis - A great asset to your team when you get in those game changing teamfights. It also gives a nice boost to your health and enough MR to still break 200. Discuss with your support on who plans on grabbing this item

Randuin's - If you find that the enemy AP champs have NOT scaled well and their AD has become more of a nuisance. Always a great item

There are many other great items to use but I find these three are the most notable

WRAITHS
Start at Wraiths and at ~1:05 place two Saplings. When the wraith's spawn at 1:40 throw 1 more on top of them and auto-attack the big wraith

BLUE
Get a leash and use your Saplings

WOLVES
Use Saplings and Arcane Smash

WRAITHS -> RED -> GOLEMS
Clear the rest of the camps, don't be afraid to spam your skills while you have blue

GANK! or return if you find yourself too low on health after the last camp

*If you find an opportunity to gank mid at lvl 4, feel free to do so with the help of Twisted Advance