Close your eyes and imagine the next 5, 10 and 20 years from now and what you want your work life to look like.
Do you see doing a few months of a niche area in residency realistically giving yourself some major career advantage? Will doing a 4th year of residency to get a few extra months to focus on something --but probably not achieve expertise in it-- afford you protected research/teaching/admin time as an attending in the future? Will it allow you to cut down your clinical work load? Will it allow you to diversify your practice in a setting outside of the ED? Will it give you an edge to get a job in an sought-after ED you otherwise couldn't break into? Will it increase your pay?
Almost certainly, the answer to all these questions will be no. Perhaps 10-15 years ago it would be "maybe." But not now. In this day and age there's a fellowship for almost every type of EM niche and the people who do those will have a major advantage in those spaces over the 4 year grad with a "focus" in residency. There are also more EM grads now and thus more people actually doing those fellowships.
EMS? There's a fellowship.
Informatics/Tech? There's a fellowship.
Crit Care? There's a fellowship.
Sports? There's a fellowship.
Sono? There's a fellowship.
Pain? There's a fellowship
HPM? There's a fellowship.
Tox? There's a fellowship.
Simulation? There's a fellowship.
Research? There's a fellowship.
Health Policy? There's a fellowship.
Peds EM? There's a fellowship.
Wilderness medicine? There's a fellowship.
Hyperbarics? There's a fellowship.
International EM? There's a fellowship.
Administrative/Safety/Quality? Three's a fellowship.
Disaster medicine? There's a fellowship.
Education? There's a fellowship.
And I'm sure there are several other examples I'm missing.
If you have to be in a geographic area for family or spousal purposes that only has a 4 year program then sure go for it and don't look back. Otherwise, I'm not sure there's any good argument left for picking a 4 year program. Your future self will thank you for picking a 3 year one.