How many lakes are there in the 4th most populous African country with a republican form of government?
SELECT COUNT(*) FROM geo_lake WHERE Country = ( SELECT T4.Code FROM ( SELECT T2.Code, T2.Population FROM encompasses AS T1 INNER JOIN country AS T2 ON T1.Country = T2.Code INNER JOIN politics AS T3 ON T1.Country = T3.Country WHERE T1.Continent = 'Africa' AND T1.Percentage = 100 AND T3.Government = 'republic' ORDER BY Population DESC LIMIT 4 ) AS T4 ORDER BY population ASC LIMIT 1 )