Yes i know yours is VAS SharePlus line.
With reference to your post mentioned
"only allowed to share 4gb";
This means that your main line subscribed plan is based on 4GB data bundle.
If you do refer to SharePlus T&C in point 4;
"Any excess usage incurred on the main line and the dependent line beyond the total bundled units"
- as this is where the limitation of this VAS end.
Since you have a total of 21GB there are inclusive of the add-on data which may not be included in the stated T&C point 4; "data bundle" of the original postpaid plan.
With the add-on data plan then there is a need to have another VAS DataShare to share the add-on plan to your other lines within the same account.
Do note that DataShare activation is effective at the point of the start of the next billing cycle period.
And DataShare is valid for those mobile lines within the same Account of the donor line.
Therefore i suggest that is to have this VAS DataShare to share the add-on data to your other lines.
Each VAS do have its limitation as it is customize in this way.
Hence there are various VAS to suit to user own customize to maximize the usage of any excess.
So there is a need to have additional VAS to supplement &or related VAS to make full use of the excess &or add-on data, example.
Since you have the add-on data to total 21GB data then another VAS DataShare as complement to share this 21GB data to your other mobile lines within the same Account.
Sorry i did not make this clear in my previous post.