This is a problem to apply the ideal gas equation.
pV = n RT => n = pV / RT
Where:
R is the Universal constant of gases = 0.082 atm*liter / K*mol
p = 4.00 atm
V = 0.050 liter
T = 27.0 + 273.15 = 300.15 K
n = pV / RT = 4.00 atm * 0.050 liter / (0.082 atm*liter/K*mol * 300.15K) = 0.0081 mol
Answer: 0.0081 mol