Step 1: Define the Library's Contents.
S1 provides a general library overview. The next logical step is to detail its contents. Sentence Q fulfills this by listing "books, journals, and digital resources."
\( \Rightarrow \) \(\underline{Q}\) follows S1.
Step 2: Identify "these resources."
Sentence S uses "these resources," referencing a previous introduction. Q explicitly introduces resources, therefore S must follow Q.
\( \Rightarrow \) partial order: S1 \(\to\) Q \(\to\) \(\underline{S}.\)
Step 3: Introduce users and their experience.
Following resources and their management (Q, S), the next logical element is the users. Sentence P introduces "Students and researchers … visit to find useful information," naturally linking to the resources.
Sentence R discusses "visitors" and the peaceful environment, referencing the visitors in P, so R follows P.
\( \Rightarrow \) order of the middle four becomes: Q \(\to\) S \(\to\) P \(\to\) R.
Step 4: Conclude with S6.
Having established contents (Q), management (S), users (P), and environment (R), S6's "Thus" offers a coherent summary.
\(\boxed{\text{Proper sequence of the four sentences: QSPR}}\)