# How to distinguish between carbon-carbon double bond lengths?

Which of the following compounds should have the longest carbon-carbon double bond?

1. $$\ce{CH2=C=CH2}$$
2. $$\ce{H3C-CH=CH2}$$
3. $$\ce{(H3C)3-C-CH=CH2}$$
4. $$\ce{(H3C)2C=CH2}$$

I know that single bond > double > triple length-wise, but how do you distinguish between double bonds?