I am having problem when I have come to a stand still and want to shift down from for instance the fourth gear to the first gear. It often will not let me do it.
This is completely normal, and many bikes are the same. If you are stopped in a high gear, you will need to rock the bike back and forth a few inches* so that the dog teeth on the gears can line up with - and drop into engagement with - the slots on the gear they are trying to engage. Keep doing that as you work your way through the gears to neutral.
You can get the same effect when the bike is on the centre stand (engine not running) - try going up and down the gears. You will need to rock the back wheel whilst keeping pressure on the shifter, and you will feel each gear drop into engagement.
The solution is to not stop in a higher gear. Make sure you are in 2nd or neutral as you come to a halt.
*Another technique is to partially release the clutch so it gently drags the primary shaft, whilst keeping pressure on the shift lever. This, too, allows the gears to move into engagement. However, it's much harder to do without the bike jumping forward. Some bikes drag their clutch a little even when the clutch lever is fully pulled in, and with those you can often go down the box without rocking. They also make a crunch when engaging first gear. ☹️