After thoroughly reading every post in this thread and googling many other sources I finally completed what everyone here is striving for.
When I start any game I first go to the forums to see any tips. Luckily I saw that I needed to save the last 4 assasinations until the end! Here is what I did thanks to everyones help!
Make sure to quicksave before you answer the phone just incase you make a mistake. You can always restart!
Step 1: Invest in Debonaire before the mission. After the mission pull out of Debonaire(80% return) and then INVEST in Redwood!!!! My Return= 300%
Step 2: Invest in Fruit shares before the mission and cashout afterwards! My Return= 50%
Step 3: Invest in Vapid AFTER you complete the mission. Cash out when you made your cut! My Return= 100%
Step 4: Invest in Gold Coast Development before the mission and pullout afterwards! My Return= 80%
Couldn't have done this without everyone else's help!
Go to hitch lift 1 with Michael(I saw someone do it with Franklin though) to pick the guy up. He should tell you about an increase in stock on the way to the airport. Before you pick the guy up though make sure all of your characters are invested in Tinkle stocks.