  We present new light curves covering 14 to 19 years of observations of four
bright proto-planetary nebulae (PPNs), all O-rich and of F spectral type. They
each display cyclical light curves with significant variations in amplitude.
All four were previously known to vary in light. Our data were combined with
published data and searched for periodicity. The results are as follows: IRAS
19475+3119 (HD 331319; 41.0 days), 17436+5003 (HD 161796; 45.2 days),
19386+0155 (101.8 days), and 18095+2704 (113.3 days). The two longer periods
are in agreement with previous studies while the two shorter periods each
reveal for the first time reveal a dominant period over these long observing
intervals. Multiple periods were also found for each object. The secondary
periods were all close to the dominant periods, with P2/P1 ranging from 0.86 to
1.06. The variations in color reveal maximum variations in T(eff) of 400 to 770
K. These variations are due to pulsations in these post-AGB objects. Maximum
seasonal light variations are all less than 0.23 mag (V), consistent for their
temperatures and periods with the results of Hrivnak et al. (2010) for 12
C-rich PPNs. For all of these PPNs, there is an inverse relationship between
period and temperature; however, there is a suggestion that the
period-temperature relationship may be somewhat steeper for the O-rich than for
the C-rich PPNs.
