Thanks, you are right, grenade does pull them away without triggering combat. So in theory I could throw grenade, enter stealth, move past the front guard's post before he comes back. But in practice, with my 3 STR and 0 throwing skill, I can't throw the grenade far enough to create time window that I can sneak in.
In the end I just opened up cheat engine and set my effective stealth skill to 300 and sneaked past the front guard. Even then I just barely made it to the shadow before his awareness indicator turns red.
IMO this challenge is not fair for a stealth character: it's either fighting 4 soldiers with no corner to hide and no chance to prepare the battle field, not entirely impossible but for a character not optimized for frontal assault will probably have to rely on save scumming for getting criticals and the enemy misses, or I need to have 300+ stealth which is simply not possible at this stage.