user28973: I'm new to Arduino, just got my starter kit, and I noticed that digital outputs 3 & 4 have 5v as HIGH level while 5 to 13 have 4.7v as HIGH level.
Is it as it suppose to be? 

user4143: Yes, that's perfectly fine.
According to the datasheet, at 5V, the minimum voltage you can expect on a HIGH output (AKA VOH) is 4.2V. 4.7V is above that, so it's fine.

user10794: Some of the answers here suggest that a reading of 4.7V is in spec (which it is). As Majenko says, 4.2V is the minimum output high voltage, at an ambient temperature of 85°C, with a load of 20 mA, and Vcc = 5V.
However with no load, and at room temperature, I measured 4.99V on all of those pins, when set to OUTPUT and HIGH on my Uno. Test code:
void setup() 
{
for (int i = 3; i <= 13; i++)
  pinMode (i, OUTPUT);
}
void loop() 
{
for (int i = 3; i <= 13; i++)
  digitalWrite (i, HIGH);
}


That was weird for me, because 1 hour ago it was showing 5v to me.

Right. What were you doing an hour ago? Did you have LEDs connected to those pins? If so, what value resistor did you have in series with them? Or were you driving a motor from the pins perhaps?

user28569: Also depends on what your driving via this output. Standard TTL is:

But CMOS will be a bit different:


28973: <p>I'm new to Arduino, just got my starter kit, and I noticed that digital outputs 3 &amp; 4 have 5v as HIGH level while 5 to 13 have 4.7v as HIGH level.
Is it as it suppose to be? </p>
 4143: <p>Yes, that's perfectly fine.</p>

<p>According to the datasheet, at 5V, the minimum voltage you can expect on a HIGH output (AKA V<sub>OH</sub>) is 4.2V. 4.7V is above that, so it's fine.</p>
 10794: <p>Some of the answers here suggest that a reading of 4.7V is in spec (which it is). As Majenko says, 4.2V is the minimum output high voltage, at an ambient temperature of 85°C, with a load of 20 mA, and Vcc = 5V.</p>

<p><em>However</em> with no load, and at room temperature, I measured 4.99V on all of those pins, when set to <code>OUTPUT</code> and <code>HIGH</code> on <em>my</em> Uno. Test code:</p>

<pre><code>void setup() 
{
for (int i = 3; i &lt;= 13; i++)
  pinMode (i, OUTPUT);
}
void loop() 
{
for (int i = 3; i &lt;= 13; i++)
  digitalWrite (i, HIGH);
}
</code></pre>

<blockquote>
  <p>That was weird for me, because 1 hour ago it was showing 5v to me.</p>
</blockquote>

<p>Right. What were you doing an hour ago? Did you have LEDs connected to those pins? If so, what value resistor did you have in series with them? Or were you driving a motor from the pins perhaps?</p>
 28569: <p>Also depends on what your driving via this output. Standard TTL is:</p>

<p><a href="https://i.stack.imgur.com/TWptO.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/TWptO.png" alt="enter image description here"></a></p>

<p>But CMOS will be a bit different:</p>

<p><a href="https://i.stack.imgur.com/DxwVx.png" rel="nofollow noreferrer"><img src="https://i.stack.imgur.com/DxwVx.png" alt="enter image description here"></a></p>
