Let's take your example where you want to create 4 archers (assuming you have unlocked the archer unit in all of your 4 barracks, of course).
The barracks used to work like this :
Every barrack will create 1 archer in 25 seconds.
Now, it works like that :
The barracks put their efforts together and create one unit all at once. Thus, an archer will be created every 25 / 4 = 6.25 seconds. 4 archers will be trained in 4 x 6.25 = 25 seconds.
Basically, if you want to create a unit, it will now take the standard training time divided by the number of barracks that have unlocked the unit. However, this doesn't change the total training time of your army. Supercell made this change so it is easier to form high-capacity troops (such as golems (30) or P.E.K.K.As (25)), and so people don't have to wait ages to make such a donation. This is especially handy during clan war times.
"I also noticed that if I fill up a queue with dragons or such, the dark barrack queue is filled up too. Are they now connected ?"
Yes. This is one of the drawbacks of the update. Now, regular troops and dark troops cannot be created at the same time ; if you are currently training a dragon and you want a golem, you'll have to wait until your dragon is trained until you can start to train your golem.
Note : Imagine you have only one dark barrack upgraded to golems (capacity of the barrack : 70). The new system doesn't allow the barracks' capacity to stack. You still cannot create more than 2 golems in that case.
Useful link : Official Clash of Clans blog post.