Answer: (1) Double eyelids and single eyelids are different manifestations of the same trait of the same organism (person), so they are called relative traits.

(2) Xiao Wang's father has double eyelids, his mother has double eyelids, and he has single eyelids, indicating that his parents' genotype is aa, so his parents have two kinds of germ cells, A or A. The combination of germ cells may be Aa, Aa, Aa respectively, and AA (double eyelids) accounts for 1/4.

(3) Xiao Li's father has double eyelids, and his genetic makeup can be aa or Aa, while Xiao Li has a single eyelid AA, which means that he inherited A's father, so his father's genetic makeup can only be AA.

(4) Xiao Zhang's parents are all single eyelids, and the children born are all single eyelids. Although Xiao Zhang's mother has become a double eyelid through plastic surgery, her genetic composition will not change, which belongs to genetic variation, so she can't have a child with double eyelids.

So the answer is:

(1) Relative? (2) 1/4(25%)? (3)Aa？ (4) no.