I was wondering if someone can help me out in finding which speedlites I can use on a Canon 4000D, as this camera does not have a centre connection on it.
Canon's own EX speedlights should all be compatible, if not fully compatible. The official word from Canon on their brochures and websites for the bodies without the sync contact is:
Speedlites other than the EX-series or non-Canon flash units cannot be used. Not all functions of the Canon EX-series flashes are supported.
In April/May 2019, Godox released firmware updates for compatibility with the T7 (aka 2000D, 1500D) and T100 (aka 3000D, 4000D) for their following units:
- TT350-C mini speedlight
- V350-C li-on mini speedlight
- TT685-C speedlight
- V860 II-C li-on speedlight
- X1T-C transmitter
- XPro-C transmitter
Yongnuo has issued similar compatibility updates for:
- YN-565C III
- YN-586C III
Nissin, Sigma, Metz
Nissin's compatibility chart lists all their gear as fully compatible with the T7.
I would also say that (obviously) single-pin/Manual-only transmitters/flashes like the Amazon Basics/Neewer TT560, Yongnuo YN-560 series, Yongnuo RF-60x triggers, LumoPro LP180, etc. won't work with a hotshoe without the sync contact.
Flashes with a built-in radio trigger, however, probably can be synced off-camera, if the transmitter can do TTL and is compatible with the new hotshoe. And any built-in optical triggering will still work.
Early testers in this dpreview thread mention incompatibility with Simpex 522, Nissin Di622, Yongnuo YN685 and RF-603, Neewer NW-670, and Godox gear. So, if you are looking at firmware-updateable 3rd-party TTL gear, I would look to see if a firmware update has been issued specifically for this issue. If not, I'd assume incompatibility unless you can find someone who reports differently.
All canon EX speedlites should work.
I have 2000D with the crippled hot-shoe too, and 580EX II and 430EX III-RT both work. The 580EX II is an older speedlite than the camera, so you are not restricted to only speedlites released after the camera was released.
I am also able to trigger both speedlites with ST-E2 optical trigger. I presume the ST-E3-RT radio trigger should also work with the speedlites having RT functionality.
Does e.g. Nissin or Metz work? I don't know. You should be very careful when purchasing third-party speedlites, because there's always a danger that they don't work with your camera.