It could just be his age, and children of the same age all develop at different rates but, to put your mind at rest, you might need to check whether or not he has a mild condition called dyspraxia. Dyspraxia specifically relates to the development of a child’s motor skills (their ability to make smooth, co-ordinated movements). It's not at all life-threatenenig, so don't worry too much.
Both boys/young men next door to me were diagnosed with this when they were children, and they have both developed into "normal" adults. Both have learned to drive, one is a car mechanic, and the other is doing A levels.
Things to look out for are:
"Movement and co-ordination
If your child has dyspraxia, they will have problems with movement and co-ordination. They may find the following difficult:
•playground activities such as hopping, jumping, running and catching or kicking a ball (they often avoid joining in because of their lack of co-ordination and may be at risk of becoming unfit)
•games including shape-sorter toys, building blocks and jigsaws
•using scissors and colouring pens (their drawings may appear to be scribbled and more childish than they should be for their age)
•fine movements such as handwriting, tying shoelaces, doing up buttons and using a knife and fork
•keeping still (they may swing or move their arms and legs a lot and find it hard to sit still)
•walking up and down stairs
•getting dressed
A child with dyspraxia may also bump into objects or drop things, and may fall over a lot. This makes them appear awkward and clumsy." Quoted from the NHS website.
Having said all this, we always worry when our children don't appear to reach development levels at the same time as others. My father was a very wise man (a teacher, as was I) who told me when I worried about my older daughter possibly being dyslexic (problems with reading), that children stay on a plateau of learning for a time, then move up to the next stage, as and when they are ready and, in my 30+ years of teaching, I found that children need different methods of educational stimulation. I used to say that this daughter used to fall over the flower pattern on the carpet when she was little but she tap dances, acts and sings in shows, and all my worries when she was tiny were completely unfounded. One child will react to listening/talking, another to reading/looking, and some will react to other, various methods of communication. A 4 year old child, in the main, has a fairly short attention span, so you will need a number of activities ready for him/her to move on to. Don't choose something that will be too long for his concentration level, and vary the type of activity you offer. Gradually you should see some changes, with patience and perseverance, and if you can find an area of activity that your child enjoys, try to develop it.
Best wishes and don't worry.......easier said than done, I know.