direct and indirect object

Please which sentence is correct and which one is wrong?
And which color is direct object, which color is indirect object?
1) I am sending her a letter.
2) I am sending a letter to her.
3) I am sending her it.
4) I am sending it to her

All the sentences are correct, though many of us would not use #3.

The direct object is in blue - it is the letter that is being sent. Some writers consider 'her' in #1 and #3 and 'to her' in #2 and #4 to be indirect objects. Others, including me, consider 'her' to be an indirect object in #1 and #3, and the object of the preposition 'to' in #2 and #4.