Match values in data frame with values in another data frame and replace former with a corresponding pattern from the other data frame

You could try:

merge(d,m, by.x="v2", by.y="v3")
  v2 v1 v4
1  A  1  a
2  A  7  a
3  B  4  b
4  B  5  b
5  C  3  c
6  C  6  c
7  E  2  e
8  E  8  e

Edit

Here is another approach, to preserve the order:

data.frame(v1=d$v1, v4=m[match(d$v2, m$v3), 2])
  v1 v4
1  1  a
2  2  e
3  3  c
4  4  b
5  5  b
6  6  c
7  7  a
8  8  e