[VladD@Kenga] [14:25:03] [~]
{0,268}$> cat xx.txt
header
data1,data2,"data3
",data4
intermediate data
data1,"data2
","data3
",data4
data1,data2,"data3
",data4

[VladD@Kenga] [14:25:26] [~]
{0,269}$> sed '/^"/{H;x;s/\n//;x;d}; x' xx.txt

header
data1,data2,"data3",data4
intermediate data
data1,"data2","data3",data4