Question (linear indexing and more)

I am trying to combine tow vectors such than if we have a={a1,a2,a3,a4} and b={b1,b2,b3,b4} .... I want to get c={a1,b1,NaN,a2,b2,NaN,a3,b3,NaN,a4,b4,NaN}.

I have attached a file of what I have done but it's not working.