You're looking for
x1 = [28 56 112 224];
y1 = [21.2 18.01 16.8 14.38];
sd_vct1 = [4.93 1.77 1.81 2.32];
instead of plotting versus ordinal position and then 'XTickLabels'
The labels will come along for the ride for free with the ticks as well as showing the real trend of the data with x...
If it really is evenly-spaced errorbars that are wanted, then
Actually, since the x1 are in power ratio, one can get the latter effect by