Of course if what you are trying to achieve is consistent indenting assuming four character tab positions what you propose won't work unless all tabs sequences start at tab positions already. If you have code such as the following (assume all white space consists of an appropriate number of tabs):

int x      = 1;
int x1     = 2;
int x12    = 3;
int x123   = 4;
int x1234  = 5;
int x12345 = 6;
[download]

then simply replacing tabs with four spaces gives:

int x        = 1;
int x1        = 2;
int x12        = 3;
int x123    = 4;
int x1234    = 5;
int x12345    = 6;
[download]

which most likely isn't what you want.