This happens in warehouse because of many reasons, but should not happen frequently and if it is happening very frequently then you need to understand the root cause to get rid of this instead of looking for blocking material in 999 storage type.
You also talked about customer places order as 4pc are still available from system point of view, so blocking material in 999 will not solve your purpose, as in sale order or in delivery stock check happens at plant level and not at WH level. So you may have to do it in both IM and WM. But I repeat it again, this may not be approach to get rid of problem, you need to think about long term perspective.
In standard there is no way to put all stock in 999 to block.
BR
Kamlakar
