First of all, try to figure out why the child is acting this way.  A new sibling? Fulfilling an adult’s expectations? Positive reinforcement of this behaviour?

Secondly, the child must become as deeply engaged as possible in some kind of work.  Remember Montssori's edict that “work is a normalising factor.”  If nothing appeals to the child, ask them to help you give lessons to others.  If a rug is needed for a lesson, have this child bring it, and unroll it for the lesson recipient.  Have this child sit with you for all your presentations.  Some will be boring to the child, some might interest the child.  Keep seeking the one activity that might capture the child’s interest – even if it is something you think they are not ready for.  Appeal, always, to their ‘higher nature’, to their ‘best self’. Ask the child what, of all the things in the class, is their favourite thing.  If you have noticed that they have mastered any piece of material, ask if they would like to be the teacher of that material to others.  Think of someone else who needs that lesson and have the ‘4-year-old toddler’ give the lesson; but first, they must give that lesson to you. Perhaps someone/everyone treats the child as he/she is behaving – as a toddler.  Treat them as if they are older, and expect them to behave like a 4-5 year-old.  Challenge them.  Try to excite them in something, anything that is more in line with their abilities.