#concatenate vertically all dataframes
dfAlldf=pd.concat([df1, df2,df3,df4], axis=1)
#sort them by the date (datetimeindex)
dfAlldf=dfAlldf.sort_index()
#group dataframe by years
grps=dfAlldf.groupby(dfAlldf.index.year)
#empty dataframe for populating with complete years
dfCompl=pd.DataFrame()
# for each group of years
for g in grps:
#if don't have any null values in year, in any column
if not any(g[1].isnull().any(axis=1)):
#concatenate in dfCompl
dfCompl=pd.concat([dfCompl, g[1]], axis=0)
# re-sort by index
dfCompl=dfCompl.sort_index()
