What depth of cut using the 0.25 end mill.?
Are you plunging down or ramping?
What feed rate have you tried.
Are you using coolant?
How do you stop the finished part moving as it breaks free?
Do you screw them down? You have holes on fished parts to do that.
I have found that if one tip on a 4 flute gets damaged the other 3 overload and fail soon after as the tooth on the other side loads up the chipped tooth.
Using a 3 flute cutter, each tooth is independent and does not have a tooth on the 'other side' so if one chips it just seems to soldier on.