  We report a detailed study of subpulse drifting in four long period pulsars.
These pulsars were observed in the Meterwavelength Single-pulse Polarimetric
Emission Survey and the presence of phase modulated subpulse drifting was
reported in each case. We have carried out longer duration and more sensitive
observations lasting 7000-12000 periods, between frequency range of 306 and 339
MHz. The drifting features were characterised in great detail including the
phase variations across the pulse window. In two pulsars J0820$-$1350 and
J1720$-$2933 the phases changed steadily across the pulse window. The pulsar
J1034$-$3224 has five components. The leading component was very weak and was
barely detectable in our observations. The four trailing components showed the
presence of subpulse drifting. The phase variations changed in alternate
components with a reversal in the sign of the gradient. This phenomenon is
known as bi-drifting. The pulsar J1555$-$3134 showed the presence of two
distinct peak frequencies of comparable strengths in the fluctuation spectrum.
The two peaks did not appear to be harmonically related and were most likely a
result of different physical processes. Additionally, the long observations
enabled us to explore the temporal variations of the drifting features. The
subpulse drifting was largely constant with time but small fluctuations around
a mean value was seen.
