Apologies -- you are correct, it is idealized to 4 minutes.  The rest of my logic still holds, however.  I do remember a few block times of around an hour due to difficulty spikes back when original Sumo was still on CN -- ASICs would join, snap up some blocks, then leave.  This lead to the difficulty spiking, and with the now-reduced hasrate, it took a long time for the rest of the network to find the next 1-2 blocks (at which point the difficulty would correct). This "situation" lead to the fork to CN-Heavy.  +10 minutes, tho not ideal, isn't near as bad as +60 minutes.