my left hand is so weak..
if i play some piano studies, my right hand can't play as fast as my right hand...
i try to exercising in scales, but it's still so hard...
it's same if i play semiquaver in pieces..
that's very bad
please give me advice...

thank u

Lots of scales in contrary motion will help I think. Make sure that your left hand position and shape is correct too. With a poorly placed hand nothing will feel as easy as it should. You could try some hand gymnastics away from the keyboard too. I posted something about these a while ago. I think that a search might find it, but I do not know how to add links. (I must find out today!)

Try some Hanon exercises - they often do a good job of helping balance up the hands.

Much as all the other advice is excellent. Do you think there may be anything else going on? By Grade 4 I would have hoped that you would be building some strength into your left hand. Is this problem confined to playing the piano only. Do you also suffer from neck pain?

Check how you are sitting at the piano as posture is very important. If necessary consult an Osteopath as they may be able to find a cause for your problems that practice alone would fail to sort out.

Jod

Hanon execises are very useful...Hanon Exercises

Often the problem isn't a question of strenght, simply a question of hand position. Try practising left hand only scales, starting in something easy (b major, or c major, for instance) and do one octave until you can scoot up and down with no problems, at any speed. This shouldn't take too long. If it is still difficult, practice it in even smaller sections. The important thing here is to discover what your left hand should look like when playing correctly. You can then apply this to the pieces you are having trouble in. I had a similar problem with my left hand, and tried fruitlessly to strengthen it, when what was needed was repositioning and awareness.
Hope this helps,
Schubertiad