684 Solo Usage Fourth Class Rate?

I noticed they have a value for 3rd class usage but not fourth class. Was it more common for fourth class so no price given?

My interpretation is that the label is misleading.
The Beecher/Wawrukiewicz domestic rate book notes on the second page of chapter 25 on third-class mail (I have the revised 2nd ed in front of me), that "on and after April 15, 1925, in most cases, third- and fourth-class matter became separated by weight, not by matter mailed.", and restated in chapter 34 on fourth-class mail.
As a convenience, companies frequently used labels intended for heavier parcels stating "fourth class" on lighter envelopes such as yours. The 1.5 cent franking would point to a single-piece, third-class letter.

Hi John that makes sense.

I will add that beginning 4-15-1925, the 1 cent per ounce parcel post rate for up to fours ounces also ended forcing that material into third class. Also that date was when single piece third class went from 1 to 1.5 cents per rate unit. Both parcel post and third class were both subject to inspection to verify qualifying content. Both required additional postage for forwarding or return. Thus using the same label made for parcel post was still useful post 4-14-1925.

Edit: As a solo usage only third class applied as there was no 1.5 cent rate in parcel post before or after 4-15-1925.