If a creature the defending player controls can’t block the creature targeted by the last ability for any reason (such as being tapped), then it doesn’t block that creature. If there’s a cost associated with having a creature block the attacking creature, the defending player isn’t forced to pay that cost, so it doesn’t have to block in that case either.
Creature type alone isn’t always enough to tell you whether a creature has four legs or not. For example, leonin have only two legs, while tigers have four legs, but both are Cats.
Generally, you look at the art of a creature card to determine whether it has four legs or not.
Magic: The Gathering · ™ & © Wizards of the Coast · Illus. Alexander Mokhov
Card data via Scryfall. Rarix is fan-made; not endorsed by the rights holders.
If the creature is known to have more than four legs—say, it’s a spider—but not all its legs are visible, it still has four or more legs. It’s a legal target for either activated ability.
If there are multiple creatures in the art that each match what the card is representing, then that creature has four or more legs as long as any one of the matching creatures in the art has four or more legs. For example, Llanowar Elves (M19) has three figures in its art, all of whom are Llanowar elves. However, none of those figures has four or more legs, so Llanowar Elves is not a valid target for Jermane’s abilities. If even one of the figures had four or more legs, then the creature would be a valid target.
If there are multiple creatures in the art, only compare the one that represents the actual creature on the card. For example, Tarmogoyf (MMA) has the tarmogoyf itself and an elk in the art, and you need only consider the tarmogoyf.
