MacPro 4,1 starting with grey screen after SMC and PRAM resets

your boot drive is Full. You may be moments away from a very spectacular crash.

you have only 26.23 GB free, and the swap file is on that drive, so just open a few more web pages and kaboom! For reference, MacOS consumes over 9GB of drive space going from a cold start to fully operational.

in addition, you have not enabled TRIM, so that drive has slowed to the speed of a rotating magnetic drive:

Performance:

    System Load: 3.86 (1 min ago) 4.73 (5 min ago) 2.75 (15 min ago)

    Nominal I/O speed: 0.03 MB/s

    File system: 15.16 seconds

    Write speed: 233 MB/s

    Read speed: 269 MB/s

use Terminal command:

sudo trimforce enable

and enter your password (which will not be echoed) and follow the directions, culminating with an automatic restart to engage.

https://www.lifewire.com/enable-trim-for-ssd-in-os-x-yosemite-2260789

.

do you have another bootable DRIVE with MacOS 10.6.8 to MacOS 13.6 on it?

if so, disconnect (pull slightly out of slot) all internal drives and boot form that different one with Option startup.

if you have not Done so lately, try a Hardware Reset:

Power down your Mac and remove the AC power cord.

Press the power-on button (with the power cord still removed) for a quarter minute to do a deep discharge of all power.

Then reconnect AC power and try to start up again.

How to download and install macOS - Apple Support

up through Sierra can be downloaded and unpacked on a Mac, and the installer is placed in the /Applications folder that can be used to make a USB-stick installer

for High Sierra, you need to use the link in the article and download through the Mac App Store, but the computer doing the download must be capable for running that version of macOS.

Create a bootable installer for macOS - Apple Support

if you name your USB-stick exactly MyVolume, those goofy long scripts can be copied from the article and Pasted directly to the Terminal command-line.

that sounds good for the display itself.

what about the graphics card?

I have had an interface (especially the higher-power DVI or its cousin HDMI) blow out, and got no display after that. I had to switch to a Mini DisplayPort adapter (to single-link DVI) to get a 1920 by 1080 picture, showing the card was good, but the interface was bad.

Do you have the diagnostic, originally on the original release disc-2?

Do you have another external or internal drive, on which you could install a macOS and boot from? or the ordinal Installer DVD, just to see if you get a picture and show more stuff is working?

these items have no place on your high-quality Macintosh computer:

User Login Items:

    [Not Loaded] AppCleaner SmartDelete (Julien Ramseier - installed 2019-02-25)

        Modern Login Item

        /Applications/AppCleaner.app/Contents/Library/LoginItems/AppCleaner SmartDelete.app

Launch Agents:

    [Not Loaded] com.avast.hub.plist (? 0 - installed )

Effective defenses against malware and ot… - Apple Community

Sorry I did not understand that your system is no longer bootable at all.

if Option startup is truly not working, then your Mac is very sick. But since you said it did draw a gray screen, I presume it is still working, but nothing suitable could be shown.

if we presume that it is only your boot drive getting flaky, you should be able to connect a different bootable drive and use Option-startup.

Sure. you can restore a time machine backup to a drive that has been erased, and has MacOS installed already.

(it MAY be possible in 10.12 to also restore the system, but that feature is being eliminated. also, to avoid carrying a damaged system forward, it is Best Practice to ALWAYS install MacOS anew.)

now you could do that:

• starting from your slower external drive

• from a USB stick BOOTABLE installer

• using your other Mac, as you just did, but move the drive to a bay when MacOS has been installed.

Some drives just seem to interfere with otherwise "usual" operation. We may never know why.

What you can do to improve repeatability is to run Disk Utility FirstAid/RepairDisk on each drive, and get the directory in the best possible shape.

You may want to get another couple of USB sticks, just to be sure the anomaly is not related to that exact USB stick.

after a few minutes with no change, that says, "no bootable drives found".

got another drive you could connect to test, or a USB stick installer, or a bootable DVD?

is your keyboard connected to a USB on the chassis, not to a display or any other USB hub?

to recognize those initial snag keys easily, there can be no intervening Hub.

Is the display actually showing a picture?

my 30-in cinema display recently conked out, and it made me nuts figuring it out. mine turned out to be the Power supply block, and the telling symptom was that the ON light (lower right corner) was not lit when the display was dark, but nominally ON.

A drive that is damaged has been know to cause ALL kinds of problems when left in place.

Some of them just refuse to go quietly.

Another way to boot to an alternate boot drive is, in the running system,

system preferences > startup disk

and specify a different connected drive as the startup drive. Then choose Restart from there.

As always, the startup disk is a parameter in the parameter RAM, and is reset by PRAM reset.

<< a no-go sign replacing the apple logo. >>

the no go "prohibitory" signs says, the drive appeared to be bootable, but on loading very early major components, one was wrong version, damaged, or missing. Boot up from this drive can not continue.