Here Oliver Cowdery initially wrote “these things” in 𝓟, but then almost immediately he corrected it to “all these things” by supralinearly inserting the all (there is no change in the level of ink flow). Either reading seems reasonable, so here Oliver was probably correcting to the reading in 𝓞 rather than editing the text. Elsewhere the text has numerous instances of both “these things” and “all these things”.
Summary: Maintain in Mosiah 4:10 the corrected reading in 𝓟: “if you believe all these things”.