The Mod DB one was blocked by my browser's (IE) smart screen filter. The idea was to scan the rar file after downloading to make doubly sure. Is the Mod Db download for the Doom 4 HUD safe to download or is there another one?

The addon in question is called the Doom 4 Emulation. Is it even possible to play this addon pack with Brutal Doom on GZ Doom?

Yes and no. I'd like the Doom 4 weapons at a later date but I'd like a Doom 4 HUD to use with Brutal Doom without the weapons: the full Brutal Doom HUD looks a bit unappealing to look at unlike the old Status Bar. and the Doom 4 HUD may fix that a bit.

It also resembles the Marathon XBLA style HUD plugin for the Aleph One engine.

You could always use the old status bar with Brutal Doom. Just turn off the alternate HUD option in GZDoom's HUD options and shrink the display by one value (- key) from the maximum, so that the HUD shows up. Optionally, you could then scale the status bar if needed.

There are many HUD mods that you can use with Brutal Doom. He does have one linked on his ModDB page, but I prefer to use a different one. I'll list a few here, maybe you'll find one you like.

Here's the one Sgt Mk IV has used in his videos of Brutal Doom. It is similar to the GZDoom built in alternative HUD, but has the extra detail for grenades and such.
http://www.moddb.com/mods/brutal-doom/addons/brutal-doom-basic-hud

Then there is the Ultimate Doom Visor (UDV). A version of this comes with Project Brutality, but it has had many updates and there are newer versions of that available. You can find this at the link below, but this is very much a major change to the HUD, and it comes with different visual styles that you can select from.
http://www.moddb.com/mods/brutal-doom/addons/doomvisor-hud-for-pa1nki113r-v51

There's NC HUD, that I found out about via Brutal Doom Johnny Edition (BDJ), which is another mod of the Brutal Doom mod, though it hasn't been updated to the latest Brutal Doom version, and is probably discontinued. The same goes for the NC HUD, too, it seems, as I can't find any recent updates.
http://www.moddb.com/games/doom-ii/addons/nc-hud-v120-compatibility-patches

And lastly, there's HXRTC HUD, which is the one I use with Brutal Doom. I like how it doesn't take up much space.
http://www.moddb.com/mods/brutal-doom/addons/na105278

The SG Mark IV one doesn't seem to work: even if I make the skins folder and place the WAD in it (widescreen because my resolution is 1920x1080), it does not work and I get the normal "minimum HUD" that GZ Doom has.

I put all of my GZ Doom and Brutal Doom files in the GOG Doom folders themselves and use a Brutal Doom PK3 desktop shortcut in the Doom 2 folder to play them and they work fine.

I only mention that just in case that's what's wrong. I also make copies of the skins folder and HUD WAD for all main Doom WAD folders to ensure it works for all of them.

I just tried it, Here's how I got it to work....
go to the ingame "HUD options: menu
make sure "stretch status bar" is set to "ON" (otherwise it's going to be small , more so at higher resolutions)

go to the "Alternative HUD" menu...
Make sure the "enable alternative HUD" is set to "OFF" I have my GZDoom separate from my Doom folder because it covers more than just Doom. I also play Heretic, Hexen & Strife through it as well.

I also use a Doom launcher, more specifically I tend to use "Doom Explorer" It lets me have an extra level of customization to my game by adding other mods, for example I may add "maps of Chaos" & add other mods to get the atmosphere I want, then create a link for that mod list

there is at least "ZDL" as another option though

I don't use a Doom launcher, but I just tried your suggestion of placing the files in the normal GZ Doom folder and set the HUD options (which were already set in the main versions) but that didn't work either.

Check that the HUD pk3 file is being loaded by GZDoom by opening the console once you have started the program and are a the main menu. If you scroll through the text you should find near the top a line that tells you it has loaded the addon.

If the HUD mod is loaded, then try using the + and - keys to change the HUD size, as it may only be visible at the maximum size screen value (but not the screen size that hides the HUD).

I haven't tried using a Doom launcher program as I create my own batch files to launch GZDoom with whichever PWAD is needed and any addons included, as well as a set configuration file for each one, as all my mods are in their own folders with their own config file.

The HUD mod is a WAD and not a PK3. I don't think it's loading when I check the console. It doesn't matter if it's in a Skins folder or not. changing the S in Skins to lowercase doesn't seem to help either.

All the other WADs load but not that one. It is called "smivhed 16-9.wad". (without quotes)

Would changing the above wad file's extension to PK3 do the trick, or would it just break it?

Doesn't matter whether it is a WAD or a PK3, they are just two different formats of addons. PK3 are actually zip files, so are easily extracted and edited.

EDIT: Well, I just downloaded it and put the file in the skins folder, and launched Brutal Doom with my batch file (after turning off the HXRTC HUD that I use, that is) and it loaded up straight away. It is something like the 8th line in the console where it says it was adding the smivhud 16-9.wad file.

I'm using the latest 64-bit dev build of GZDoom (v2.3 pre-222, released today), but this HUD mod has been around for quite some time, so I wouldn't have thought it was a version issue, especially if you are using the version that comes with the Hell on Earth pack.

I am using 1.8.02, but I have 2.2.0 as well just in case.

Tried with 2.2.0 just now but it still didn't work. It is not appearing in the console.

Where is your skins folder? There should only be one skins folder, located alongside the GZDoom executable. Put the HUD WAD/PK3 inside that folder and GZDoom will always load it, regardless of any command line parameters from shortcuts or batch files. See my screenshot.

There is a skins folder in all my Doom folders including GZ Doom.