What do you mean OS3 on your machine?

Do the apps say that they are OS4 compatible?

Go to the app store on your ipod and look at the reviews for the apps in question. If other people are having similar problems this is the vehicle they tend to use to vent their feelings.
I have one or two apps that have struggled to run since the os upgrade, because of the reviews, I know that I’m not the only person having issues.

Oh I thought you had os4 on your ipod.

They should still work.

“Should” is different than “does”. I’ve broken enough stuff with updates over the last quarter-century to know that difference all too well I also find it funny since this is exactly* the sort of problem Apple fanbois belittle PCs for having, but I have a weird sense of humor.

Seriously though, Apple did a major upgrade of the OS and it’s only natural that some older apps would be broken and some have been improperly updated to the new OS which is practically a ground-up rewrite as opposed to just a minor revision/patch.