Search and Replace using awk

Hi, I am really confused with this problem that I am facing .
I have a file that contains entries in the form :

option1 Value1
option2 Value2
option3 Value3
option4 Value4
.
.
.

I want to search for the keyword "option4" and replace "value4" by another value, say "value5".
My main problem is, i have no clue about the values, I just know the name of the "options" available. So, i have to search for the option and then somehow replace the corresponding value.
Is there any way using awk to do this?
Thanks!!

Can you show us what have you tried so far and where you are stuck?

Regards

echo option4 Value4 | sed 's/option4.*/option4 <new value>/'