Um, I think you would need to merge. But you can unmount the partition in order to do so.
Boot into the LiveUSB of Zorin OS (the trial version). This way, you can unmount your
partition in order to make changes.
If you are referring to sda3 and sda4, that is good. In order to expand one partition into another, they must be adjacent to each other.
The Downside Is that sda3 would need to be clean free space first. You would need to move all data you wish to keep off of it, then restore all data once the merge is complete.
Another option would be to automount that partition on boot. Then redirect your downloads to that partition or other data you wish to save there.
Either way - it is a lot of work. And that small 20gig space is going to haunt you, if you do not increase that space. No matter what work-arounds you do.
If it was me... I would just clear space, merge all free space partitions I could, then reinstall Zorin OS. Do it Right Once, save yourself from multiple headaches later.
A reinstall of Zorin OS can be done pretty painfully by planning ahead.