I am really surprised that gladiola plants will survive in zone 4. Wow, one learns something every day. I've been digging them up every year here in zone 5. I shall have to investigate this some more.

If they are hardy to your zone, then I would think that now would be okay. It will be interesting to hear what others say.

I just researched and found this:

Storing over winter: Gladiola bulbs will come back year after year in regions where the ground does not freeze, but they are not winter-hardy in northern climates, so if you want to keep them going, dig them up after frost has blackened their leaves and store them in a frost-free place.

However, most experts advise buying new ones each year, as gladiola corms can be prone to damage from insects and fungal diseases.

If you decide to overwinter them, be sure to remove any remaining stem after it has dried up, and treat corms with a combination fungicide and insecticide dust.

I have them on the south side of the house and have never dug them!! They multiply every year. I just need to know if this is the time to divide them??? I'm actually in Zone 5 but don't know how to change it. Denver Col. Thanks

I would dig them up and replant in spring. But to answer your question, I think the best time to dig them up and replant would be in early spring or after they have died back. If you do it now, be quick so they have a chance to adjust before winter.
I'm in zone 5 and a lot of mine come back year after year. But I get some that occasionally rot. So, to play it safe, I always dig up a handful of my favorites and store for the winter in case it's a very bad winter and they take a big hit.

Ours return each year, but it seems each year they are less vigorous. (This is in the high desert west, where we have about four or five months of winter, that include several episodes of freezing temps.) when I lived in So Cal, we had glads that seemed to propogate themselves, but the new plants were never as large or hardy as the originals.

We are in NW Michigan and it definitely freezes here although we do have good snow cover. When I want to move mine, I do it in the spring, feeding them at the same time. Left in place with no soil ammendments, they don't seem to grow as vigorously.