Hi. I was wondering if E400 and E410 APs can co exist together and if the clients will easily roam between the two APs. Also would there be any downside in combining these two APs on one network. Thanks.
Sure they can coexist... they're very similar AP's. From a software/firmware standpoint they're essentially the same. The hardware is very similar as well, with the 410 having a couple extra features... most notably AC wave 2 support which introduces MU-MIMO... but you'll need to have clients that support AC wave 2 to get any benefits from that feature. The 410 has 1dB better antenna gain on both 2.4 and 5ghz, TX power are the same for both... they both should have approx. the same coverage characteristics... the 410 might be hair better depending on your regulatory area.
I can't think of any downsides mixing them... although, If the majority of your clients aren't wave 2, you could save yourself some money and just deploy 400's (like a hospitality/coffee shop/hotel situation). If you want to plan for the future and know your clients are moving to wave 2 as well (a place with controlled IT, industrial, hospital, school, etc), then you'd want to focus on the 410.