I would go for the 4-plex. I wish there was some of those in my city, we only have probably 200-300 duplexes here (2 of which we own) and I wouldn't do another one.
The economies of scale just arn't there with the duplex (or at least not in my city).
The main advantages is that it only consumes 1 of our 10 allowable personal loans (yet yields 2 properties), and that there is a shared structure and roof.
You still have multiple of everything else... water heaters, HVAC, plumbing systems, etc. The roof isn't really much of a bargain either because it is roughly 2x as big as a SFH.
The big disadvantage is that you're locking yourself in with what are typically lower-quality tenants (more temporary minded, roomates, lower income, etc) than a SFH.
Granted all of these gripes are specific to my market, where I've determined I can get roughly the same value in a SFH and attract better quality tenants at the same time, there's no use going for duplexes.
however, if these were 4-unit buildings I could understand how the benefits could start to outweigh. Sadly there are 0 of these type structures here for me to ever find out.