REmember that
If two triangles are similar, then its corresponding sides are proportional an this ratio is called the scale factor
In this problem
we have that
triangle DEF and triangle DAB are similar
that means
DA/DE=AB/EF=BD/FD=scale factor
so
DE=4 units
DA=8 units
therefore
scale factor=8/4=2
answer is
scale factor is 2