I am really leaning toward getting one of these for my new Doublestar Patrol rifle that has nothing but iron sights on it now.  But, the mount that comes with it looks pretty low compared to other mounts I see sold separately.  Would I need to add a block or spacer under it to make it comfortable to use or not::

BTW, I am a little taller than average and like to shoot an AR with my head up high....

The M4 comes with a mount that includes a riser.  So you can do an absolute co-witness or use the riser for a lower 1/3 co-witness.  If you were to get a Bobro QD mount you'd use that same riser on it for a lower 1/3 co-witness.  The LaRue LT659 on the other hand is one solid piece so you don't need that riser as it's set at a lower 1/3 co-witness.  I have one on a Bobra w/ the riser and one on a LaRue.  The LaRue sits a tad bit higher.

Some mistakes in that response that need to be cleared up...

The M4 comes with the QRP2 mount which is a 2 piece mount, a base and a spacer. For an standard AR, you will need to use the spacer to get an absolute co-witness. If you want a lower 1/3 co-witness, you need to go with one of the aftermarket options.

The Larue LT659 mount comes in 3 heights. The tallest with give you a lower 1/3 co-witness. The LT659NV is the medium height mount and will give you an absolute co-witness. They make that mount because Mil-Issue NV mounts need the sight to be at absolute co-witness height. LaRue also make the LT659 HK specifically deigned for weapons like the HK-416 which have a higher rail height.

I ran the QRP2 mount for about a year and over all, it's a good mount. I did not like the big knob on the side but it did make it easy to install and remove the mount without tools. I just didn't like how far it stuck out and found it could get caught up on web gear. Aimpoint now makes the LRP base which has a quick release lever but for the price, you might as well step up to a LaRue or Bobro mount IMO.

I now use the LaRue LT 659NV mount as I prefer an absolute co-witness and like how it allows me to mount the sight a little further forward due to the cantilever design of the mount