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The fourth part of the high galactic latitude search for millisecond period pulsars in the right ascension range between 13$^h$ and 18$^h$ using the Arecibo telescope as a transit instrument has been completed. In the survey conducted between April and August 1993 we have found one millisecond period pulsar (PSR J1640+22, $P$=3.16 ms, in a 180--day circular orbit with a low--mass companion), seven slow pulsar candidates (four already confirmed during the April and August sessions) and two known pulsars (PSR B1633+24, PSR B1530+27). The total area searched at 430 MHz exceeded 400 square degrees. However, about 25--30\% of data were severely corrupted by radio frequency interference, and more than 60\% of data had some background interference of varying strength. The steadily growing contribution of man--made interference to data has seriously impacted the current search sensitivity. We summarize the results of this latest survey along with the three previous surveys conducted by the authors in a similar manner.