Pin 4 Stuck Low

I have got a problem with a Duemilanove board in that pin 4 is permanently LOW.
I haven't used it for a few years and as far as I know pin 4 used to work ok.

Any suggestions as to how I can troubleshoot this? It is not a programming issue as all the other pins when set to HIGH will light an LED ok(ish) - I say ish because pin2 when set LOW still gives 2.1v but when set HIGH gives the same 4.8V as the other pins. I don't know whether these are linked problems.

Any thoughts would be most welcome.

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Here is the code to test the ability of the pins to go low.
The circuit is +5v on Arduino to 360 ohm resistor to +LED -LED to pin.

void setup() {
pinMode(0, OUTPUT);
pinMode(1, OUTPUT);
pinMode(2, OUTPUT);
pinMode(3, OUTPUT);
pinMode(4, OUTPUT);
pinMode(5, OUTPUT);
pinMode(6, OUTPUT);
pinMode(7, OUTPUT);
pinMode(8, OUTPUT);
pinMode(9, OUTPUT);
pinMode(10, OUTPUT);
pinMode(11, OUTPUT);
pinMode(12, OUTPUT);
digitalWrite(0,LOW);
digitalWrite(1,LOW);
digitalWrite(2,LOW);
digitalWrite(3,LOW);
digitalWrite(4,HIGH);
digitalWrite(5,LOW);
digitalWrite(6,HIGH);
digitalWrite(7,LOW);
digitalWrite(8,LOW);
digitalWrite(9,LOW);
digitalWrite(10,LOW);
digitalWrite(11,LOW);
digitalWrite(12,LOW);

}
void loop() {
}

(deleted)

Thanks for getting back to me here are the pin reading with nothing attached. The anomalies are pins 4 & 2. Where do the pins get their signal from is it directly from the ATMEGA chip? I have reseated the IC but it made no difference.

All pins set LOW
Pin - Voltage +5V to pin
0 - 4.71
1 - 4.86
2 - 3.01 *
3 - 4.88
4 - 4.92
5 - 4.92
6 - 4.71 *
7 - 4.90
8 - 4.91
9 - 4.91
10 - 4.91
11 - 4.90
12 - 4.93
13 - 4.90
And when all pins are set HIGH
Pin - Voltage GND to pin
0 - 4.91
1 - 4.91
2 - 4.94
3 - 4.91
4 - 0.06 *
5 - 4.91
6-13 - 4.91-4.94

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OK here are the voltages all referenced to GND
Thanks again

HIGH Ground to pin
0 - 4.91
1 - 4.92
2 - 4.94
3 - 4.92
4 - 0.06
5 - 4.91
6 - 4.93
7 - 4.93
8 - 4.93
9 - 4.90
10 - 4.93
11 - 4.93
12 - 4.93
13 - 4.93
LOW Ground to pin
0 - 0.23
1 - 0.08
2 - 1.92
3 - 0.06
4 - 0.00
5 - 0.02
6 - 0.25
7 - 0.03
8 - 0.04
9 - 0.02
10 - 0.02
11 - 0.02
12 - 0.01
13 - 0.01

(deleted)

That's great - thanks for the advice.