Formula help - fill Blank cell with antoher cell info

[CL1000]

I need a formula that if a cell is empty then it will used info from another cell, but then if I add text to that cell it will show that info util it is deleted and is blank again so it would revert back to the formula.

[TABLE="width: 500"]
<tbody>[TR]
[TD="align: center"][/TD]
[TD="align: center"]A[/TD]
[TD="align: center"]B[/TD]
[/TR]
[TR]
[TD]1[/TD]
[TD]table[/TD]
[TD]50[/TD]
[/TR]
[TR]
[TD]2[/TD]
[TD]chairs[/TD]
[TD]300[/TD]
[/TR]
[TR]
[TD]3[/TD]
[TD]none[/TD]
[TD]0[/TD]
[/TR]
[TR]
[TD]4[/TD]
[TD]if blank then use A3
[/TD]
[TD][/TD]
[/TR]
[TR]
[TD]5[/TD]
[TD]

if blank then use A3​

[/TD]
[TD][/TD]
[/TR]
</tbody>[/TABLE]

​

 

[ParamRay]

= if( isblank( b4 ) , b3 , b4 )