I'm making a twin-sized porch swing. Should I use 4 chains such that two of them connect together - thus resulting in only two from the ceiling, or use 4 chains and all 4 connect to the ceiling separately?
The chain configuration does not just affect the amount of support. It also controls the angle of the seat of the swing as it moves through its arc.
If you use a single hook on each end, the seat is always perpendicular to a line that extends from the center of the seat to the hook.
If you use two hooks on each end, the seat will track differently, depending on the length of the chain and the relative positioning of the hooks. One example is a parallel configuration.
Some configurations simply will not work well (e.g. if the space between the hooks is much wider than the space between the attachment points on the swing). Also, chains of different length will cause the seat angle to change (tip forward or backward).
As several comments point out, it is essential that whatever attachment system you use, the chains and the ceiling supports must be strong enough to hold the swing, the people and then the load must be multiplied by at least two or more to account for ther moving load.