Yes, if you bought V4 from DAZ, you should be able to just download the files again. You shouldn't have to buy it again.

And you can download zip files instead of using the install manager. The trick is to recognize which files to download. They're all labeled DAZ Studio now, even if they're meant for Poser. After awhile, you'll get to recognize which ones to download. You don't need the files with PwrLoader or StudioCF in the names. Some products have Ps or Ds in the file names; you want the one with Ps.

There are some exceptions. Occasionally, you'll come across a zip file that's corrupt. Because DAZ doesn't really care about Poser any more, these products don't get fixed. If you find one of those, using the install manager is the best solution.

You can put in a ticket, but expect a looooong wait. I contacted support about a corrupt file, and they did eventually fix it. It took them over a year, but they did fix it.

When they asked me to rate the interaction, I honestly didn't know what to do. I mean, they did fix my problem. But over a year later doesn't seem very reasonable.