In relation to my original comment, I had to split my gedcom into 6 before it would upload. The total people in my tree were less than 4000. I'm updating this question in case others are having the same problem. I split my tree at my great grandparents and uploaded the split gedcom files individually to WikiTree. I used the Gedcom Splitter at http://www.llenrup.com/splitter
which is a legitimate site.
I got my mother (at a different residence and hence a different internet connection) to upload the original (4000 ppl) gedcom file to her WikiTree site and she had the same issue. She tried a gedcom with only 800 people in it and it worked perfectly. We are both in a regional area in Queensland, Australia and I wonder if the internet settings and speeds may have played a part.
So, if you are looking for a reason behind errors occuring when uploading a gedcom file to WikiTree, try splitting the file. The smaller the amount of people uploaded in a Gedcom, the more likely the uploads will be successful.