with a thunderbolt usb4 can I use usb3.x external drives for backup?

I agree. :) and IMHO, backup drives. (Time Machine or some other backup mode) don't really need to be all that fast anyways. my main reason for saying that is that even though I use TM, I have never needed to restore from TM over all of the time that I've been using Macs. and also, after the initial backup, incremental backups are pretty quick. and as a footnote, my TM disk is a glacially slow HDD. (western digital "my passport")

The differences between USB-3 and USB 4 are very small, and for most purposes, are negligible.

Remember that DRIVE speed is the determining factor is most cases. ThunderBolt-3 or 4 provides a large PIPE, but a larger pipe can not make a slow drive transfer its data any faster. In general, ThunderBolt 3 or 4 are limited to about 1500-ish M Bytes/sec maximum transfer speeds, and most drives are still slower that that.

Time machine does not benefit from a drive that is fast. Time machine does it backing up at low property in the background, so paying extra for a fast drive for Time machine buys you nothing.

yes, they will work fine. just don't expect the TB3 drives to run a TB4 speeds.

I think I misunderstood. USB3 (if that's what you have) drives will require an USB-C to USB Adapter to work on your TB4 equipped Mac.