If you don't control a creature with power 4 or greater immediately after Garruk's Uprising enters, its first ability won't trigger. If you don't control one as the ability resolves, you don't draw a card. They don't have to be the same creature both times, however.

The first ability of Garruk's Uprising has you draw just one card, no matter how many creatures you control with power 4 or greater.

If one or more static abilities that apply to a creature entering change its power, those abilities are considered when determining whether Garruk's Uprising's last ability triggers. The same is true for replacement effects that apply to it, such as entering with one or more +1/+1 counters or entering as a copy of another creature.

Once the last ability of Garruk's Uprising has triggered, lowering the power of the creature or removing it from the battlefield won't stop you from drawing a card.