I am new to D&D and have been searching for what attack modifier is referring to. IS it the strength modifier (for fighter +4)? I am using the Starter Kit. 4e
I don't have the Starter Kit but since an attack modifier entry is in the core manuals I'm supposing the developers used the same terminology.
An attack modifier is each of a series of different situational modifiers that apply to an attack roll, such as the +1 bonus for charging, the -2 penalty from concealment, the -2 to hit prone opponents with a ranged weapon, the +2 from combat advantage, or the -2 for long range (the list is not exhaustive). The key here is situational.
Less strictly speaking, attack modifier could be the whole modifier to the attack roll, after factoring in all the bonuses and maluses from all sources. I guess I could tell you what's the case by looking at the snippet of text where the expression is used.
The Starter Kit is essentially a simplified version of D&D4, IIRC, and like many other things it compiles a number of things into the attack modifier.
If you could give us a little more detail, as in exactly where it says this, we could help you a little more, but here's a quick breakdown:
What most people popularly refer to as "attack modifier" is the ultimate change to the numbers (though the textbook definition is usually just the situational things) that determine whether or not you will hit with an attack. This factors in a number of sources: