#You might try passing actual types instead of strings.
import pandas as pd
from datetime import datetime
headers = ['col1', 'col2', 'col3', 'col4']
dtypes = [datetime, datetime, str, float]
pd.read_csv(file, sep=' ', header=None, names=headers, dtype=dtypes)
#But it's going to be really hard to diagnose this without any of your
#data to tinker with. And really, you probably want pandas to parse
#the the dates into TimeStamps, so that might be:
pd.read_csv(file, sep=' ', header=None, names=headers, parse_dates=True)