Activated abilities contain a colon. They're generally written "[Cost]: [Effect]." Some keywords are activated abilities and will have colons in their reminder text.

If the enchanted creature's power isn't 4 or greater when the end step begins, the last ability won't trigger at all.

Arachnus Web's triggered ability checks at the beginning of each end step, not just yours.

If Arachnus Web's last ability triggers, but the enchanted creature's power is reduced to 3 or less before the ability resolves, the ability will have no effect. Arachnus Web won't be destroyed.

Triggered abilities (starting with "when," "whenever," or "at") are unaffected.

If Arachnus Web enters the battlefield attached to an attacking or blocking creature (due to Arachnus Spinner's ability, for example), that creature will continue to attack or block.