I am currently reading a text book on distributed computing systems that includes a short introduction to $\lambda$-calculus. There is an example of evaluating the sequence $(((if \space \space true) \space \space 4) \space \space 5)$ which is written below.
$\space\space(((\lambda b.\lambda t.\lambda e.((b \space t) \space e) \space \lambda x.\lambda y.x) \space 4) \space 5) $
$\space\space((\lambda t.\lambda e ((\lambda x.\lambda y.x \space \space t) \space e) \space 4) \space 5) $
$\space\space(\lambda e ((\lambda x.\lambda y.x \space \space 4) \space \space e) \space \space 5) $
$\space\space((\lambda x.\lambda y.x \space \space 4) \space \space 5) $
$\space\space(\lambda y.4 \space \space 5) $
$\space\space 4$
The author has used $\beta$-reduction on each line and I can follow up until the second to last reduction. Could someone explain how we get from line 4 to line 6?