Sparky 4 based on Debian 9 “Stretch” has been moved from stable to oldstable line now and is still supported.

What you should do now?

If you would like to keep Sparky 4 based on Stretch, you have to:
1. Check Debian repositories to make sure your os uses ‘stretch’ repos, at the /etc/apt/sources.list
2. Install the latest version of ‘sparky4-apt’ package which points to Sparky oldstable repos now:
sudo apt update
sudo apt install sparky-apt sparky4-apt

3. Then make system upgrade:
sudo apt update
sudo apt full-upgrade

Sparky Upgrade Tool should install the latest ‘sparky4-apt’ package before performing the system upgrade, but if it doesn’t, close the Upgrade Tool and do it manually in a terminal emulator, as above.

More info about Sparky Oldstable repositories can be found at the Wiki page: repository_oldstable

If you would like to upgrade Sparky 4 to Sparky 5 – it is possible, but I recommend to make fresh Sparky 5 installation.
By the way, if you decide to make such upgrade, do:
1. Change Debian repos from ‘stretch’ to ‘buster’ or keep ‘stable’ at the /etc/apt/sources.list
2. Temporary change Sparky repos from ‘oldstable’ to ‘stable’ at the /etc/apt/sources.d/sparky-oldstable.list if your system uses Sparky ‘oldstable’ repos already, and refresh package list:
sudo apt update
3. Install the latest version of ‘sparky5-apt’ package which has to remove ‘sparky4-apt’:
sudo apt update
sudo apt install sparky-apt sparky5-apt

4. Then upgrade the system:
sudo apt update
sudo apt full-upgrade

If any problem:
sudo apt install -f

Let me know if you find any problem with your system upgrade, please.