Some congregations have a minhag of reading a section from parshat Nasso at the beginning of the month of Nssan. The section read is related to the offerings that each tribal leader gave to the dedication of the mishkan. Some congregations read this section from a Torah.
I know that there is a rule of tircha detzibbur regarding congregational Torah reading. I.e. - we try to avoid or mitigate a pause for the congregation by verifying that the Torah scrolls are rolled to the right place. This explains why this Shabbat, we use 3 Sifrei Torah.
For those shuls that make a congregational reading from Nasso, as described above (usually, this is done at the end of davening), do they need a 4th Sefer Torah to avoid "tircha detzibur", or does it not apply in this case, and they can roll one of the 3 (sounds like the 2nd one which ws read from Pinchas would make the most sense) used in the morning service?