first: convert grams to moles using the molar mass of butane from the periodic table
4.00 g C4H10 x (1 mol C4H10)/(58.12 g C4H10) = 0.068823 mol C4H10
second: convert moles to molecules (using avogadros number)
0.068823 mol C4H10 x (6.022×10^23 molecules C4H10)/(1 mol C4H10) = 4.14452×10^22 molecules C4H10
third: there are 4 carbon atoms in 1 butane molecule, so use the following ratio:
4.14452×10^22 molecules of C4H10 x (4 atoms of C)/(1 molecule of C4H10)
= 1.66×10^23 C atoms (3 significant digits)
Hope this helps