Wow 4# at 8 wks he's going to be a big boy... He will probably outgrow a playpen before too long.
I bought an xpen. I love it. I bought the 24in high one and it came with a bag for transportation. It's great for travel, in a yard w/o fence. We have a houseboat and go out on sandbars for day and overnight trips. I like to put them in the pen, then I don't have to worry about other dogs or them wandering off. It's good size but you can actually make it any size or shape you want.