I went to upload my sketch to my Uno (r3), but unusually, the Upload Console returned this error:
avrdude: ser_open(): can't open device "\\.\COM4": Access is denied.
Just started seeing this, cannot upload.
There might be another device has already connected to that port. It could even be a terminal emulator which is left connected from last debugging session.
Otherwise, make sure that your UNO is really presented on COM4 by checking with "Device Manager". if not, change the port where it is available.
You have selected the wrong COM port. Go to Tools -> Port -> Select the right port. If you do not know the port number disconnect the Arduino, open the same menu again and look what is listed there, write down if you can't remember, now plug your Arduino back in and select the option that wasn't there before.
I'm using a windows 10 and had this happen randomly. I closed several other applications and it started working again.