Being that you are dealing with such a small spatial area, we can get the four coordinate pairs using a couple of Euclidean equations. Unfortunately, GIS.SE doesn't support LaTeX equations, so I am going to assume you know basic geometry equations like slope, point-slope form, and the euclidean distance formula.
To better illustrate what is being done, refer to this image:
The goal is to find the coordinates of r, s, t, and u.
First, let's state the givens.
A (lat: 10.767008, lng: 106.665884)
B (lat: 10.767715, lng: 106.667151)
r-s, t-u = 4 metres or roughly .000036036 decimal degrees at this latitude
(if you need a more accurate way of determining this, there are plenty of posts on the subject)
Second, calculate the perpendicular slope of line A-B, we call it M.
Similar to slope equation, however with perpendicular slope your numerator is:
x_1 - x_2
And your denomimator is:
y_2 - y_1
M = (106.665884 - 106.667151)/(10.767715 - 10.767008)
M = -1.7920792
Third, we write out our equations for our segments, solving for the y coordinates:
y_r = M(x_r - 106.667151) + 10.767715
y_s = M(x_s - 106.667151) + 10.767715
y_t = M(x_t - 106.665884) + 10.767008
y_u = M(x_u - 106.665884) + 10.767008
Fourth, accounting for slope being positive or negative, we then write out the equations solving for the x coordinate of our unknown points. If the slope is negative, the x coordinate of s and u will be the result of the negative difference. If positive, the x coordinate of s and u will be the positive sum. Since our slope is negative, that means:
x_s = 106.667151 - (0.000018018 / sqrt(1 + (-1.7920792)^2))
x_s = 106.667139
x_r = 106.667151 + (0.000018018 / sqrt(1 + (-1.7920792)^2))
x_r = 106.667163
x_t = 106.665884 + (0.000018018 / sqrt(1 + (-1.7920792)^2))
x_t = 106.665896
x_u = 106.665884 - (0.000018018 / sqrt(1 + (-1.7920792)^2))
x_u = 106.665872
Fifth, now that we have our x coordinates, we can plug the values back into our segment equations and get our y coordinates.
y_r = -1.7920792 * (106.667163 - 106.667151) + 10.767715
y_r = 10.7676935
y_s = -1.7920792 * (106.667139 - 106.667151) + 10.767715
y_s = 10.7677365
y_t = -1.7920792 * (106.665896 - 106.665884) + 10.767008
y_t = 10.7669865
y_u = -1.7920792 * (106.665872 - 106.665884) + 10.767008
y_u = 10.7670295
So the coordinates to create your polygon would be:
r (lat: 10.7676935, lng: 106.667163)
s (lat: 10.7677365, lng: 106.667139)
t (lat: 10.7669865, lng: 106.665896)
u (lat: 10.7670295, lng: 106.665872)
Understand, they may be slightly off due to the assumption of metre to decimal degree length at this latitude, and values were rounded off.