cd /tmp
git clone https://framagit.org/pkg/obextra/postgresql
cd post*/trunk
makepkg
sudo pacman -U post*.xz
This will build a copy of the pkg with libraries/dependencies you have currently installed so it will work (93.4% sure it will :)
And give some time for Eric to rebuild the pkg, which will replace this one with a higher version (from -3 it might be -5) if arch hasn't bumped it up higher.
Looking at the PKGBUILD it doesn't require a specific version of icu, whatever is available, so it will build with the new icu.