The SamsungGalaxyAce doesn't let me download apps!Even with a huge SD Card I have a Samsung Galaxy Ace and a 4GB SD card, I only have one app that is whatsapp, and When I try to download another app That is 40MG(a lot less than 4GB), it says insufficient Space, but I go to settings and it says that I have 3.69GB of space on SD card, and on the phone's space that comes with it says 20 MG available, so I have 3.69GB,and 20MG, and it doesn't let me download apps!What do I do?
If the device info is correct here then your device's internal memory is 158MB (quite low IMO) in total out of which you can use Diskinfo(see Userdata in it) and Diskusage (both &lt;1MB individually)to see how much space is used and is available. You can then uninstall any app that you may have installed at some time but don't remember. 

However, I don't see much scope here since the issue would arise again sooner or later considering the (very low) storage set by OEM. 

Note that the storage used by apps is /data which is different from the external SD card that you've of any size. Some apps until Android 4.2+ can transfer data files (.obb) into external SD card rather easily but it is a painful job in above versions now.