Trouble understanding this from Ciaffone/Reuben's book

[7n7]

Reading Reuben/Ciaffone's <u>PL &amp; NL Poker</u>, and am having trouble understanding the following example from page 22.

"...suppose there is $5k in the pot. You are first to act with unlimited funds. Sid follows w/unlimited funds and Dave is last to speak holding $8k. If you are very strong, you should bet more than $4k. This way Sid knows that if Dave raises all-in, you would not be able reraise. He [Sid] is thus more likely to call. "

My question: why does Sid know this and why couldn't/wouldn't you reraise to build a side pot?

It then goes on to say that if you're weak, the play is to bet less than $4k b/c Sid may pass b/c he could be facing 2 raises now.

Somebody help school the Omaha noob please.