There are two Ss. Peter & Paul churches in Chicago. One appears to be Lithuanian and the other one on South Paulina is in fact Polish. The LDS library catalog does not make a distinction. The Polish records appear on FHL films 1763433 and 1763434.
With respect to the baptismal records, the quality of the handwriting was excellent which leads to more accurate indexing. It is peculiar that the fourth volume begins in October 1915 and repeats records from the third volume for the same time period. For purposes of the index, the names for volume 4 were copied from volume 3 and then paginated to be consistent with volume 4. So don't be surprised if a child's name shows up twice in the same church. While filming ended before the start of 1916, both volumes 3 and 4 continue on beyond 1915. The duplication of records presumably continues as well.
The death index is based on the section with good handwriting. I would venture to guess that these were copied from other volumes. Why? There is another section containing 1913-1916 deaths which is not as neatly written and which specifies, in many cases, a more precise age at the time of death. Be aware that you probably can find other images of the 1913-1916 deaths.
The online description of image groups for this parish includes some items that are for the Lithuanian parish-- a cataloging problem.