If I have 4 gears on the field attached to 4 units, when I cast "Arise!", how many sand soldier tokens can I get?

4 tokens — as long as all four of those gears are actually Equipment.

Arise! reads "Play a 2 [M] Sand Soldier unit token for each Equipment you control," and Equipment means Gear with the Equipment tag (Rule 150). Plain Gear like a Gold token or a non-Equipment relic doesn't count, so if even one of your four isn't Equipment you'd only get 3.

Attachment is irrelevant to the count — attached Equipment is still Equipment, still Gear, and still controlled by you (FAQ #2018), and nothing limits how many can be attached. The only real snag is the back half: "Then do this: Ready up to two of them" doesn't actually function as written, because the tokens aren't played until after the spell resolves, so there's nothing on the board to ready (FAQ #346).