4 Av, commemorates the beginning of the 52-day period in which Nehemiah rebuilt the walls of ancient Jerusalem. This period will finish on the 25th of ELUL. Nehemiah faced tremendous resistance returning to and rebuilding the city - no different today!
This time overlaps with the Nine Days when we are mourning the destruction of Jerusalem, when the walls were breached on the 17th of Tammuz and the Temple was set on fire on the 9th of Av, burning right through the 10th of Av.
Let's make an EXTRA EFFORT to advocate for Jerusalem, the Eternal Capital of the Jewish People and for G-d's House - a House of prayer for ALL nations. May we see the ancient and Eternal Jerusalem be rebuilt in its full glory soon in our days.
On a personal level, this is an auspicious time to pray for your own "walls" - (may it be a lack of protection bodily [ie immune system, etc], physically, psychologically [lack of resolve against negative resistance] or spiritually, to be healed and restored. Use your personal prayers to also assist you to petition for the Shalom (wholeness) of Jerusalem.