Mega Man 4’s passwords are very similar to the previous two games except that they do not remember how many Energy Tanks you possessed at the time you got the password. 
Such a shame, as storing this number is as easy as adding one more dot to the grid.

The password scheme works like this: There are ten items that the passwords record: eight Master Weapons, and two Adapters. 
Capcom paired these items into five groups of two items each. 
Each pair has only one dot on the grid associated with it. 
For each pair, there are four dots possible: one of the items acquired, the other acquired, neither acquired, or both acquired. 

Here’s the chart.

Weapon/Item	Both	Neither	One
Toad Man: Rain Flush	A1 	A2 	B1
Bright Man: Flash Stopper	A1 	A2 	C2
Weapon/Item	Both	Neither	One
Pharaoh Man: Pharaoh Shot	A4 	A3 	B4
Drill Man: Drill Bombs	A4 	A3 	C3
Weapon/Item	Both	Neither	One
Ring Man: Ring Boomerang	B5 	A5 	B6
Dust Man: Dust Crusher	B5 	A5 	C5
Weapon/Item	Both	Neither	One
Skull Man: Skull Shield	E2 	D1 	F2
Dive Man: Dive Missiles	E2 	D1 	E1
Weapon/Item	Both	Neither	One
Wire Adapter	F3 	E3 	D4
Balloon Adapter	F3 	E3 	D3

Now, before the password will work, you need to add one more dot. 
This dot stands for how many items/weapons you have, total. 
There are eleven possibilities for this final dot: ten items plus one dot for no items. 

Here’s that chart:

0 	1 	2 	3 	4 	5 	6 	7 	8 	9 	10
F5 	A6 	B2 	B3 	C1 	C4 	C6 	D2 	E4 	E6 	F1


So to construct a password, punch in the dots for which, if any, of the Adapters you wish to possess, and which Master Weapons you want to have (the stages related to those Robot Masters will be completed, of course). 
Remember to double up the dots into the single dot when necessary, and don’t forget a single dot for the number of items you possess. 
At this point, you are done; there’s no way to skip Dr. Cossack’s fortress and go on to Wily’s. Sorry.