How can I cut some digits from a number?

How can I cut some digits from a number like this 0.0000005566?

How can I keep only the first 4 decimal digits?

I use vpa but this return something else.

b=0.0000005566
vpa(b,4)

ans =

5.566*10^(-7)

examples:

0.0000345->0

0.001234->0.0012

12.4234234->12.4234

121313.334242->121313.3342

[James Tursa]

Can you give some examples of input and desired output?

Answer 1

[Oleg Komarov]

n = 121313.334242;
fix(n*10000)/10000

[sadel]

hahaha I was just thinking the same!!!!