Big maps with so much object render

Hi, actually I’m making a project in unreal is my first time. I wannna make a map 4km x 1km and need use so much object for this.
My question is when add so much objects this is very lag approx 10k objects.
How can optimize my map for avoid the lag??
Help me please, I’m desperate

you can start with Cull Distance Volume:

and Actor Merging (4.12):