Strings have been a subject of endless debate on this forum...
I prefer D'Addario EJ19 Bluegrass Medium Lights on my D-25M.
Welcome aboard Modeana!
I can just about guarantee you that D4's shipped with Guild's L350 lights.
THE closest thing available today (it uses a different gauge "G") would be D'Addario Ej-16's, uncoated pb lights 12-53, the EJ-19 would be the next thing to try to get a bit more bass thump.
If those aren't bright enough consider 80/20 formulations.
If they're
too bright coateds might offer a bit of "taming down" even though they're supposed to be "sonically identical" to uncoateds these days.
"FWIW" for 20 years I was a D'A EJ-16 'purist' since they were Guild's OEM supplier when D4's were in production and all
regular production dreads got 'em from
at least '96 through '04.
So they're a good place to start, and if you want to replicate what it came with EXACTLY, buy a D'A PB025 single to sub into the set for the "G"; you will then have the identical gauges to Guild's L350 strings used during that era.
I still do that with my dreadnoughts, but in the last year I had an epiphany while trying to tame down the maple jangle of my F65ce and so now wholeheartedly support recommendations to try a few sets to discover what
you truly like.
And I HATE changing strins so I'm sympathetic about that issue.
The only additional advice I'll give is that those were designed to use lights and going to mediums
could actually surprise you and be counterproductive by putting too much tension oin the top.
Also lights will help preserve the neckset angle for as long as possible.
D4's were originally offered without cases as an economy measure.
Any case sized for a 5" deep dreadnought should fit a D4; Guild only offered 2 cases for dreadnoughts, and they were both used for both the archbacked and flat-backed models, the difference was in the top ("Deluxe" had an arched
top for extra strength) and lining and latch count.
Both of my Guild dread cases were made by TKL, just to mention another source.