Can i download ue 4.17.2 now that 4.21 has been released?

due to shader level 4 card i want to download the 4.17.2 version because the 4.21.1 wont work without a shader level 5 card

Yes. Go to epic games launcher, navigate to library, select plus on unreal engine, there is a drop-down at the top right of the new icon that lets you select versions.