Build image with lede-modhostapd on router with 4MB flash

I cant build image with lede-modhostapd on my router with 4MB flash (, can everybody tell me step by step to build? Thank you very much and sorry for my bad English.

Make sure you are using a config which would normally make a small enough image. The added code should add only a kB or so to the size of the squashfs.

I can build my own image with Lede image builder, i remove some unnecessary packages, so my df -h on my router here:
Can i add the lede-modhostapd by build image with lede image builder?

You need to use the complete build system to compile in the new source code. Image Builder works by combining binary files that are already compiled into one big file that is the image. That is the final step the complete build system does.

I have a problem while building:

Did you follow this guide?
Which OS are you using for building?