Having a scale of 0.25 in = 4 feet means that every 0.25 inches in the model represent 4 feet in the real room. Using this knowledge and the length and width of the actual room we can find the values we need to build the model.
Let's start with the room's length using the rule of three:
4 feet----------0.25 inches
20 feet---------x
According to the rule of three x is given by:
If we make the same process for the model's width:
4 feet----------0.25 inches
12 feet---------w
So for the model we have that x=1.25 inches and w=0.75 inches