Era 4 Preparation

Level up your Laboratory. Level 20 is needed to get Era 4 Coupling by the end of Era 4 without a plus account. Level 17 is needed with a plus account.
Be sure to save enough money (5,300,000) to upgrade your Engine shed right after we hit Era 4.
Get Coupling. If you won't have Coupling by Era 4 but you can get it less than a day into Era 4, you should finish the research for it. 1 to 2 days is more questionable but I wouldn't blame you for getting Coupling. More than 2 days, IMO forget about Coupling and save your research points for Era 4.
Have at least 4-5 free tracks to begin Era 4. Or connect to all Era 4 RGs by using a Map Of Goods.