documentclass{book}
usepackage{array}
usepackage{fp}
begin{document}
begin{tabular}{|*{4}{c|}}
hline
a & b & c & d  \
hline
multicolumn{3}{|c|}{a} & b \
hline
multicolumn{2}{|>{centeringarraybackslash}p{  0.5textwidth}|}{a} & b & c\
hline
end{tabular}
end{document}

documentclass{book}
usepackage{array}
usepackage{fp}
begin{document}
begin{tabular}{|*{4}{c|}}
hline
a & b & c & d  \
hline
multicolumn{3}{|c|}{a} & b \
hline
FPmulone{2}{2}
FPdivtwo{1}{2}
multicolumn{one}{|>{centeringarraybackslash}p{  twotextwidth}|}{a} & b & c\
hline
end{tabular}
end{document}