Huh? Where is the problem? What doesn't work for you and how does that show?
Exact mountings and location of the gam on your computer, please

http://www.thatfleminggent.com/ultima/u4download.html

OS is Vista. Game is on C:/Ultima4. When I try to enter a city, the dos input comes up over the game and all I can do is exit.

Edit: DosFreak, that's where I got it from, but I'd love step-by step instructions.

exact mountings please, e.g. what do you type when you start dosbox?
Also make sure that the files are not write protected - a common problem if you copy the game off a collection CD.
Also please try the version DosFreak pointed at to make sure that your game is not borked somehow

mount d c:/ultima4 -t cdrom -usecd 3
mount c c:/
C:
cd/ultima4
Ultima

Is that right?

Works fine.

DOSBox is no different than MS-DOS except where DOSBox is different (DOSBox only commands such as "mount").

If you have no experience with MS-DOS or command line then you need to have at least some experience before trying to play games in DOSBox. Best bet would be to read the DOSBox README (included with DOSBox) and mess around.

1. Download
2. Unzip
3. Create c:\oldgames
4. Create c:\oldgames\install
5. Move unzipped files to to c:\oldgames\install
6. Remove Read-Only attribute on files. (highlight all files in Windows, go to properties, uncheck Read-only)
6. Execute dosbox and mount c:\oldgames
ex "mount c c:\oldgames"
7. go to c: drive
8. go to install directory.
9 Type "install.bat"
10.Ultima 4 should load right after the install.

If you can pester the above linked website to create a DOSBox packaged version.

yes, follow dosfreaks instructions. the way you are doing it is WRONG

Did it exactly like that just now, but still it doesn't work. When I enter a town, it brings the DOS command line up at the top of the screen and all I can do is exit.

Edit: DosFreak, I read all of the stuff that comes up when you type "intro" and all of the different intros, and I understood everything. I have no idea how to fix this problem.

I have no idea, too. Since I just did these steps and it works fine for me. Are you actually doing this in Dosbox? Or with Dosbox version 0.74?

.74, why?

because something is wrong at your end. it works fine everywhere else...
Are you still using this mount command:
mount d c:/ultima4 -t cdrom -usecd 3

Because that is just wrong on about a thousand levels

Can you post the contents of your dosbox.conf?

Go to DosBox 0.74 Option in DOSBox, it will open the file in Notepad. Cut and paste the contents here

Edit: Works now. I think I probably mistyped something the first time. Thank you both for your help.

follow dosfreaks instructions to the LETTER and BEFORE you execute install.bat, please make a screenshot of your Dosbox with the dosbox capture keycombination CTRL-F5 and post that here.
I have no idea why that doesn't work on your side. it works perfect here