vierjährig
four-year-old
Having an age of four years.
Das vierjährige Kind konnte schon seinen Namen schreiben.
The four-year-old child could already write his name.

four-year-old / four-year
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four-year-old
Having an age of four years.
Das vierjährige Kind konnte schon seinen Namen schreiben.
The four-year-old child could already write his name.

Adjectives like 'vierjährig' (four-year-old/four-year) are formed from a number and a unit of time (here: year) with the ending '-ig'. This pattern is very productive, for example also in 'zweitägig' (lasting two days) or 'einstündig' (lasting one hour).
Wir machen eine dreitägige Wanderung in den Alpen.
We are going on a three-day hike in the Alps.
You distinguish the meanings by the noun: if 'vierjährig' refers to a living being (child, animal), it is about age. If it refers to a thing (contract, training program, term of office), it is about duration.
Unser vierjähriger Sohn kommt bald in den Kindergarten.
Our four-year-old son will soon start kindergarten.
To state the age of a person or animal, 'vierjährig' (four-year-old) is used only before a noun. When the adjective stands alone after the verb, you use the construction 'vier Jahre alt' (four years old).
Ein vierjähriges Pferd ist noch sehr jung.
A four-year-old horse is still very young.
In the meaning 'lasting four years', the adjective is often combined with nouns such as 'Amtszeit' (term of office), 'Ausbildung' (training program), 'Studium' (degree program/studies), 'Plan' (plan), or 'Vertrag' (contract).
Nach dem vierjährigen Studium suchte er sofort einen Job.
After the four-year degree program, he immediately looked for a job.
annual / yearly
'jährlich' and 'vierjährig' look similar, but they do not mean the same thing. 'jährlich' says that something happens once per year or is valid for one year. 'vierjährig' means either four years old or lasting four years. The risk of confusion comes from the part 'jährig'. The important point is: 'jährlich' describes a repetition or validity per year. 'vierjährig' describes an age or a duration of four years.

Meine Nachbarin hat eine vierjährige Tochter.
My neighbor has a four-year-old daughter.
Die Kontrolle der Heizung findet jährlich im Herbst statt.
The heating system is inspected annually in the fall.