@JohnD You can 'install' LL4.x to that same partition - yes but you want to format it.
If your /home resides on another partition you 'can' leave it in place -** Although there will be some existing files that could interfere.
^^ so if sda1 is where root (/) resides and say sda2 is /home, you can format sda1 and install LL4.x
*If /home and root are both on the same partition - it cannot be done in this manner and you will loose data*I don't think you can just install over top without formatting (and if you could -- I WOULD NOT)
There are difference between LL3.x and LL4.x that could cause issues (even with /home)
You could maybe clean up any 'user' files just keeping the data and hoping...
What may be an option in this scenario - if both are on separate partitions.. You could format root and install LL4.x, when creating a user create a different user than in LL3x. If you had John in LL3.x make it JohnD in LL4.x then you could move all the 'data' files from John to JohnD (old to new).
Then clean up/delete the remaining old profile (directory).
Regardless - I would suggest backing up the data you dont want to loose - should you inadvertently click something or what have you... Its easier to loose data than get it back...