How to solve SD card problems in a smartphone

First of all in a 4GB SD card available will be somewhere around 3 and as you said you used 2 of that and then we left out with MB count. Operating system never allows memory till the last byte, so if they say 4GB you can use almost 2.5 to 3 to perfection.

Better option is to be buy 8 or 16 GB based on what your device supports and also see that apps installed in internal memory not on SD. This makes the folder repeated every time.

If your phone suddenly pops message to mount SD card when it's already mounted, then it might be a problem of the SD card slot. Try to avoid frequent mounting of different SD cards in the same phone. If you have to transfer or watch something from other device, then tranfer via Bluetooth or other sharing apps, e.g. - Xender, Zapia etc.

You said you store videos upto 2GB space. Do you keep your apps on SD card too? Then it will take some more space which you might have omitted when counting. In my phone I often find some miscellaneous files also taking space such as downloaded documents etc. A 4GB card gives nearly 3.6 GB effective space. But in my opinion, buy at least 16GB card, as 4GB is not enough now a days for a smartphone. Keep eyes on the online markets, you may get one 16GB card at Rs. 330 approx if you are lucky.

Additionally scan your SD card for virus and spamware, as they might take some space and also slow down your phone.

Such wrong reporting of storage space on a memory card occurs when the card has any issue. I would advice you to check the card on your PC. It should report the correct space left on your card.
Also check with some other memory card on your phone. If you still get the mounting issues, it is better to hard reset your device. Please note that all your installed apps and other files will be lost. Take a backup of your important files before proceeding.