Half Marathon time x weight

To figure out your expected time:

expected_time = average_time * (1 + dm/m)

dm = change in your weight = -4 lbs
m = your weight = 169 lbs
average_time = 97.17 minutes

expected_time = 97.17 * (1 - 4 / 169) = 94:52

So, that's 2 minutes and 18 seconds faster.

The above equation assumes our bodies work in a linear fashion. For example, if you gain 10% in weight, it takes 10% more energy. It's very unlikely that the body is that efficient, and in practice I find a quadratic equation works better:

expected_time = average_time * (1 + dm/m)^2
expected_time = 92:37
That's 4 minutes and 33 seconds faster.

Bottom line: Given your current weight and current time, dropping 4 lbs you can expect to decrease your time somewhere in the range of 2:18 to 4:33.