I see old film holders made by Graflex described as type 5 & type 4. Which of these is appropriate to my Pacemaker 4x5?
I have seen those for the GRAFLEX back which have the groove. I should avoid these, correct?

Right, the "Graflex" holders are designed to engage a metal lip or ridge on the back of the camera, which is both a light trap and a latch to keep the holder in place laterally. Holders for the "Graphic" and later "Graflok" backs have a groove instead of that lip, which engages a raised ridge on the film holder.

I've not heard of differences among holders designed for the "Graphic" back that would unfit the earlier ones for use with "Pacemaker" cameras, though I know relatively little about the history of cutfilm holders.

In the U.S., one usually finds either Graflex "Riteway" or Lisco "Fidelity" holders for "Graphic" backs. The ones I've purchased used, over four decades, never were sorted or displayed by age or model sequence.

My impression is that the "Graphic" back is, or was, Graflex's implementation of a widely-used spring back design. I know the "Graphic" holders will work with a Burke & James "Speed Press," and my guess is that they'd work with just about any '40s-through-'60s vintage hand-and-stand camera.

Okay just to beat the dead horse....stay away from groovy holders that say "Graflex film holder"

The differences between type 4 and type 5 are slight. and before I bore you with the details both will work, the type 5 works better.

Type 4s are slightly shorter and have one ridge near the film aperture.

Type 5's are longer and have a second light trap (read raised boss) that wraps around the edge of the camera. All holders since WWII are type 5.

The benefit is that the holder sticks out a bit farther and you can grab it better. Early on there were some large portrait cameras with 4x5 reducing backs that couldn't handle the extra lenght, but I've yet to see a Speed Graphic with a Graphic or Gralock back that wouldn't take either, and I have Speeds or Crowns from 1912 to 1973.

Simply put, "Graflex" holders with the groove down the sides only fit "Graflex" backs. They do NOT fit spring backs or Graflok backs. Any "Graphic" holder will fit the Pacemaker if it doesn't have a "Graflex" back, and yes, they were available on Pacemakers, though rare. In addition to the points made by Les about Type 4 vs Type 5 holders, the Type 5 is was made to be useable with infrared film. Any modern, as in new, brand of cut film holder will fit if you have a spring or Graflok back. Confused ? So am I.