Ok, let's hear about it.
Those 2-stroke engines we use for model application have standard mufflers mostly with single baffle.

I have seen several people running 2-stroke engines with 4-stroke muffler (long multiple baffles). Most of them told me no problem doing this. Some told me not to.

I think it is just fine to use 4-stroke muffler (pipe such as Macs, Bolly, Hatori, Asano etc).
I am trying to use tuned pipe with header, but I already have several Hatori, Asano, ES carbon mufflers for 4-stroke engines.

If I use 4-stroke muffler, will I damage the 2-stroke engine?
What do you think?

I don't think the 4-stroke pipe is likely to damage your engine, but you won't have a tuned system. So you will loose some power compared to a tuned 2-stroke pipe.

Just make sure the muffler volume is big enough for you engine so you don't get too much back pressure. The volume of the first chamber in your pipe is important here...

On the other hand... your nick is PatternFlyer... so I guess you will want all the power you can get??? That means you will have to use a proper tuned (and muffled) pipe...