Why don't all cylinders have priming lines?
The airplane maker will not normally install any more primer lines than necessary. In warm environments it's not usually necessary to prime more than one cylinder, so you will see airplanes operated in the southern US with single primer lines more often than in the northern US or Canada.  My old '68 Cardinal only had a single primer port because it had been imported from the US that way (I was in Toronto at the time), and it was murder to get it started even in moderately cold weather without preheat. I had primers run to all 4 cylinders by my mechanic.
Also, you will often see one of the threaded holes at the intake port on a cylinder being used as a tap-off for a manifold pressure gauge, another reason for stopping at 3 lines.
When priming, the objective is to get a good coating of raw fuel in the intake pipes close to the port, that will take 5 or 10 seconds to fully evaporate after starting, to provide that temporary extra-rich mixture.  But not too much.
Over-priming can start a fire when so much gas is squirted in it starts to collect in the intake pipes and drain down into the airbox.  A backfire into the intake system during starting, because an intake valve doesn't quite seal, can ignite the fuel in the airbox, and potentially fuel that is dripping into the cowling.
That's why, when cranking and you hear a &quot;fump!&quot; sound under the cowling, keep cranking!  You need to try to draw any ignited raw fuel back into the intake.  Only stop cranking if you start to see smoke gushing out, and then it's time to leave and tell the owner he can have his plane back (or try to fight the fire if you want).
It's when you start to smell gasoline that you know you've over done it.  Best to stop and wait a while for all the gas to evaporate.
A worse habit is throttle pumping to spray fuel from the carb's accelerator pump into the intake.  You might do it when you start a warm engine; it's not necessary to prime with the primer and little shot of throttle while cranking is enough.
But it's critical to only pump the throttle during cranking. If you pump it while things are stopped, gasoline runs straight down into the airbox immediately.  Probably most cowl fires are from people pumping throttle several times, waiting a few seconds, then starting, then it's &quot;Fump!&quot;.... &quot;do you smell something?&quot;.