Need help with using an entity from a different project (error: 10481)!

I need to use a entity 'task3' from a different project 'lab3' the file name I need it from is also 'task3'. My current project is 'lab4' . I am trying to port map but have been unsuccessful please help em regarding this. The following is the current code for lab4.

library ieee;
use ieee.std_logic_1164.all;
use ieee.numeric_std.all;
library lab3;
use lab3.all;

entity top is
port(cin: in std_logic;
a,b: in std_logic_vector(3 downto 0);
sum: out std_logic_vector(3 downto 0);
cout,seg: out std_logic);
end top;

architecture behaviour of top is
component full_adder is
port(x,y,cin: in std_logic;
s,cout: out std_logic);
end component;
signal c0,c1,c2,c3 : std_logic;
begin
adder0: full_adder
port map(x => a(0), y => b(0), cin => cin, s => sum(0), cout => c0);
adder1: full_adder
port map(x => a(1), y => b(1), cin => c0, s => sum(1), cout => c1);
adder2: full_adder
port map(x => a(2), y => b(2), cin => c1, s => sum(2), cout => c2);
adder3: full_adder
port map(x => a(3), y => b(3), cin => c2, s => sum(3), cout => c3);
cout <= c3;
hexto7seg: entity lab3.task3
port map(a => sum, f => seg);
end behaviour;