1. (noun) last 2 million years

• age of man
• quaternary period

2. (noun) the cardinal number that is the sum of three and one

• 4
• four
• foursome
• iv
• little joe
• quadruplet
• quartet
• quatern
• quaternion
• quaternity
• tetrad

3. (adjective) consisting of or especially arranged in sets of four

• quaternate

4. (adjective) coming next after the third and just before the fifth in position or time or degree or magnitude

• 4th
• fourth

1. (noun) the cardinal number that is the sum of three and one

• digit
• figure

1. (adjective) consisting of or especially arranged in sets of four

• multiple

2. (adjective) coming next after the third and just before the fifth in position or time or degree or magnitude

• ordinal