The 23andMe V4 chip is 23andMe's fourth microarray chip and is the one now in use. It replaced their V3 chip in November 2013. It is a completely custom Illumina Genotyping BeadChip.
Microarray chips test a small percentage of SNPs on the human genome. Each letter the DNA code for the human genome (A, C, T, or G) is called a base pair. This one tests about 602,300 SNPs.
SNPs per Million Base Pairs – Heatmap
The V4 chip tests about 602,300 base pairs across the human genome. This heatmap shows how those SNPs are distributed across the 22 autosomal chromosomes and the X-chromosome. The ranges are for counts of tested SNPs in blocks of 1 million base pairs.