It was extremely difficult to spot this w1 (=an extinct bird) w2 (=in the middle of, surrounded by) the bushes, but once the notorious gunman from Perfection,Nev. w3 Gummer saw it, the bird's w4 was inevitable.

w1,w2,w3,w4 are four-letter words
w2 and w3 are jointly composed of 8 distinct letters.

If the letters in the words are replaced by nine digits (distinct digits representing distinct letters)- the following equations come true:

w3+w1=w2
w3-w1=w4

q1: What digit in the alphametics is not represented by a letter?
q2: What is the bracketed word in the title ?

Hint (not mandatory):
4 968301 9401 94Y 625301 Y628 3I5

The above senseless, albeit grammatically correct, sentence may be of help.