Multi attack by default increase your primary skill by 50% MH. 4× Multi attack means 4×50%= 2. 2+1= 3×. That means your primary skill gets multiplied by 3x. Say you dealt 150% MH. That means 150%×3=450% MH for primary skill and not only that but hero point can multiply that by 2 into 900% and further multiply that by 50% with skilled mythic.
Primary skills can get multi attack and extra attack but special skills don't unless you use a mythic called Discordance.
If you use Discordance, this means the 3× buff to MH% dmg applies to special skills such as orb.
Also if it's a primary skill and multi attack was applied, you have more animation. For example flintlock would shoot 2-4 more bullets.wand would shoot 2-4 more barrage and other skill that shoot more than one bolt/bullet/turret. If it's special skill, the 3× more dmg to the special skill is there but no changes to animation like no two orbs if you using discordance but maybe with multi shot.
For cronos' LOHKO build, with discordance his arrow for some reason got buffed by multi attack even though with Discordance rules, multi attack and extra attack works on special skills only instead of the previous primary skills only and that was a bug but that bug allowed him to deal 2000% MH or more per arrow and you can imagine how overpowered that would have been in patch 2.0. actually no, multi attack didn't affect his arrows so I don't know why he added multi attack.
Maybe it was for his guided shot defense for his one shot arrow build the arrow had 1050% MH because it swapped multi shots value with arrow so arrow get that 1050% MH and multi shot get 100% MH or so. Because of things like skilled though and timewarp , he easily gets over 2000% MH for single shot arrow using Discordance mythic and one shot many opponent.