krabbeln
to crawl
To move on all fours, as babies do before they learn to walk.
Unser Sohn hat mit acht Monaten angefangen zu krabbeln.
Our son started crawling at eight months.

to crawl
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to crawl
To move on all fours, as babies do before they learn to walk.
Unser Sohn hat mit acht Monaten angefangen zu krabbeln.
Our son started crawling at eight months.

As a verb of movement, 'krabbeln' (to crawl) forms the perfect tense with the auxiliary verb 'sein' (to be), not with 'haben' (to have). This applies to all meanings of the word.
Das Baby ist schon durch das ganze Wohnzimmer gekrabbelt.
The baby has already crawled through the whole living room.
The exact meaning of 'krabbeln' (to crawl) depends strongly on the subject of the sentence. If it is a baby, it means moving on all fours. With insects, it means their typical movement.
Mein kleiner Neffe kann schon schneller krabbeln als ich laufen kann.
My little nephew can already crawl faster than I can walk.
When 'krabbeln' describes an itchy feeling, the subject is often an object, such as an item of clothing. It can also be used impersonally with 'es' (it), with the person feeling it in the dative case.
Dieser neue Schal krabbelt an meinem Hals.
This new scarf itches against my neck.
four
'auf allen vieren' (on all fours) and 'krabbeln' both fit a movement using hands and knees. 'auf allen vieren' mainly describes the body position. 'krabbeln' is the appropriate verb when especially a baby or small child moves this way. So you can say that a baby 'krabbelt'; while doing so, it is 'auf allen vieren'. The expressions can be confused because they often show the same situation. However, 'krabbeln' sounds more natural when the movement is the focus.

Das Baby krabbelt schon sicher vom Teppich bis zur Tür.
The baby is already crawling confidently from the rug to the door.
Beim Spielen kroch der Junge auf allen vieren durch das Zimmer.
While playing, the boy crawled through the room on all fours.