Numbers 4:20 gives the reason for the warning in verse 18 — the Kohathites must not see the holy things uncovered or they will die.

Numbers 18:5 commands guarding the sanctuary to prevent wrath on Israel — directly continuing the same rationale for Levitical duties.

Exodus 19:21 warns the people not to break through to gaze at God lest they perish — same warning against approaching holy presence improperly.

Leviticus 10:2 shows the fatal consequence of Nadab and Abihu's unauthorized fire — a direct example of the death warned against in Numbers 4:18.

1 Samuel 6:19 records God striking men for looking into the ark — exactly the kind of improper viewing forbidden to Kohathites.

2 Samuel 6:7 shows God's anger killing Uzzah for touching the ark — the very outcome Numbers 4:18 warns against.