I've used the DW Spot monitor mentioned in the thread John mentioned for two different installs. It is limited to four views but works well. The limitation I initially found was that it said it supported cameras up to 5MP, but it didn't support 4MP. The cameras that were displayed on the unit were 1080P so that wasn't a problem. DW tech support initially told me it was supported until I corrected them and they said they would change their documentation. I haven't tried a 4MP since then but I don't believe a firmware update changed that issue and if it did, then their documentation isn't really accurate or clear enough in opinion.
If it's only for four cameras the NLSS seems expensive. The interface on the NLSS seems nicer though and there seems to be more features available on the NLSS.