If I understand the requirements correctly, I don't think bakunin's code work with string like n3d5. Since both alpha and numeric characters are present, it prints both a number and a string:

Code:

0.25,0.11
0.59,10.25
nd,8.46
10.46,0.00n3d5
-12.46,-8.55

and like RavinderSingh13's code, there aren't supposed to be any digits after the decimal point when converting field #4.

I came up with the following before seeing RudiC's code. It uses similar logic but approaches it a little bit differently:

Code:

awk -F, -v OFS=, '
function format(field, digits_after_radix) {
	gsub(/^ *| *$/, "", $field)
	return sprintf("%"($field~/[[:alpha:]]/?"s":"0."digits_after_radix"f"),
		    $field)
}
{	print format(2, 2), format(4, 0)
}' xxx > yyy

Note also that this code strips leading and trailing <space>s from the fields before printing them, but preserves any internal blanks in alphanumeric strings. R. Singh's code strips leading and trailing whitespace characters from field #2, but only leading whitespace characters from field #4 (with an input file that contains four fields). RudiC's code strips all <space> characters from both fields. And, bakunin's code strips the same characters that R. Singh's code strips. With your sample data, none of this matters, but if your real data contains trailing whitespace characters in field #4 or if field #2 or #4 contains spaces in the middle of the field, you'll need to adjust whatever code you choose to give the results you want.

If xxx contains:

Code:

.112      ,    0.25         ,    0.48      ,    .112
5.232    ,    0.5852     ,    10.25    ,    10.25
8.455    ,    nd            ,    10         ,    8.455
n.d.       ,    10.4558   ,    aa_a     ,    n3d5
-8.55     ,    -12.458    ,    80.985  ,    -8.55
-8.55     ,    not a number    ,    80.985  ,    Not 1 number only

the above code produces the output:

Code:

0.25,0
0.59,10
nd,8
10.46,n3d5
-12.46,-9
not a number,Not 1 number only

With the awk on macOS Sierra version 10.12.2, RavinderSingh13's code with the above input file, produces the output:

Code:

0.25,0.11
0.59,10.25
nd,8.46
10.46,n3d5
-12.46,-8.55
not a number,0.00

which I do not understand. I would have expected the n3d5 in the 4th output line to be 0.00 instead since (like the last field on the last line) that field contains both digits and alphabetics.