$解:(1)把A坐標(biāo)代入反比例函數(shù)$
$4=\frac {k}{1},k=4$
$∴y=\frac {4}{x}$
$把B坐標(biāo)代入反比例函數(shù)$
$-1=\frac {4}{n},n=-4$
$∴B(-4,-1)$
$把A,B坐標(biāo)代入一次函數(shù)$
$\begin{cases}{ a+b=4 }\ \\ {-4a+b=-1\ } \end{cases}$
$解得\begin{cases}{ a=1 }\ \\ { b=3 } \end{cases}$
$∴y=x+3$
$(2)x\lt -4或0\lt x\lt 1$
$(3)設(shè)點(diǎn)C的坐標(biāo)為(c,\frac{4}{c}),D(d,0)$
$①以AC、、BD為對(duì)角線$
$則\begin{cases}{ 1+c=-4+d\ }\ \\ { 4+\frac {4}{c}=-1+0 } \end{cases}解得 \begin{cases}{ c=-\frac {4}{5} }\ \\ { d=\frac {21}{5} } \end{cases}$
$∴\frac{4}{c}=-5,∴C(-\frac{4}{5},-5)$
$②以BC、AD為對(duì)角線,同理有\(zhòng)frac {4}{c}=5$
$∴C(\frac{4}{5},5)$
$③以AB、CD為對(duì)角線,同理有\(zhòng)frac{4}{c}=3$
$∴C(\frac{4}{3},3)$
$綜上,當(dāng)點(diǎn)C的坐標(biāo)為(-\frac{4}{5},-5)或(\frac{4}{5},5)或(\frac{4}{3},3)時(shí),$
$以A、B、C、D為頂點(diǎn)的四邊形是平行四邊形$