The +4 or +3 would be any given modifier. Like if my weapon does 1d6 damage PLUS damage equal to my Strength Modifier, and my Str Mod was "+4" then I would write "1d6+4". The dice roller just adds that number onto the end automatically and gives it to you in the result.
Or if I wanted to make a skill check, I would put in "1d20+[skill check modifier]" like if I wanted to roll Stealth, and my Stealth skill is at +7, I would write "1d20+7"
If I wanted to roll at advantage, I would roll "2d20+7" but could not rely on the total result; because that will just add both d20's together. You have to look at the results page and figure out which result is the higher number and then add the +7 manually.
EDIT: Although I suppose in that last example, if you were rolling at advantage, there's no reason to include the modifier at all since you'll have to add them manually in the end anyway xD
EDIT 2 Extra Example: Looking at your character sheet, let's say Alwin wants to attack a target with his Clan Dagger. You need to roll to hit first, so you would roll "1d20+5" since your weapon's "to hit" modifier is +5.
Then if the GM tells you that that roll hits, you roll the weapon's damage. In this case, the Clan Dagger's damage roll is "1d4+2" so that's what you would roll. And the damage type is labeled as piercing or can be bludgeoning. For this level 1 adventure, damage type probably isn't all that important, but if you play higher levels, that will probably be progressively more important to keep in mind as enemies might resist or be vulnerable to certain damage types.
Or if I wanted to make a skill check, I would put in "1d20+[skill check modifier]" like if I wanted to roll Stealth, and my Stealth skill is at +7, I would write "1d20+7"
If I wanted to roll at advantage, I would roll "2d20+7" but could not rely on the total result; because that will just add both d20's together. You have to look at the results page and figure out which result is the higher number and then add the +7 manually.
EDIT: Although I suppose in that last example, if you were rolling at advantage, there's no reason to include the modifier at all since you'll have to add them manually in the end anyway xD
EDIT 2 Extra Example: Looking at your character sheet, let's say Alwin wants to attack a target with his Clan Dagger. You need to roll to hit first, so you would roll "1d20+5" since your weapon's "to hit" modifier is +5.
Then if the GM tells you that that roll hits, you roll the weapon's damage. In this case, the Clan Dagger's damage roll is "1d4+2" so that's what you would roll. And the damage type is labeled as piercing or can be bludgeoning. For this level 1 adventure, damage type probably isn't all that important, but if you play higher levels, that will probably be progressively more important to keep in mind as enemies might resist or be vulnerable to certain damage types.