To do this all you need to do is load the image and all of its pages with the Load method. After that you can loop through however many pages, saving each page as a separate TIF file with the Save method. Below is a code sample for VB.NET. For more detailed information on how to use the Load and Save methods, consult the help file.
img = codecs.Load(Application.StartupPath + "\..\combined.TIF", 0, CodecsLoadByteOrder.BgrOrGray, 1, 4)
codecs.Save(img, Application.StartupPath + "\..\1.tif", RasterImageFormat.Tif, 24, 1, 1, 1, CodecsSavePageMode.Overwrite)
codecs.Save(img, Application.StartupPath + "\..\2.tif", RasterImageFormat.Tif, 24, 2, 2, 1, CodecsSavePageMode.Overwrite)
codecs.Save(img, Application.StartupPath + "\..\3.tif", RasterImageFormat.Tif, 24, 3, 3, 1, CodecsSavePageMode.Overwrite)
codecs.Save(img, Application.StartupPath + "\..\4.tif", RasterImageFormat.Tif, 24, 4, 4, 1, CodecsSavePageMode.Overwrite)