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.
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