umm where is the fifth starktech prototype? i killed all 4 and all the bosses.. but the "?" isnt appearing on the minimap

I dont see the challenge tab. when i press L.

im lvl 21.. i see legendary quests though

To see the challenge tab click on the waypoint and it should be the far right tab after the terminals etc. although if you're level 21 now then just focus on legends rows which as you said are under the L hotkey.

oh i see, im doin xdefense now... do i get partied with other ppl around my level?