No need to sand it. Weldon 4 is a slow setting solvent. That means you are going to have plenty of time to line things up.
Some additional points to consider:
First of all, you are not "gluing" the pieces together. Acrylic is chemically bonded. For successful bonds, you will need to make sure both surfaces soften and then are mated together under pressure. I can't say how long that would take with #4 since we never use it because it takes too long to set. Weldon 3 however sets very quickly leaving no room for error. If you are not going to enter your bond into a beauty pageant, then simple saw cut surfaces and #4 are good enough. I am assuming you are bonding flats to flats. If that is the case, then you should have no issues. It gets a bit more difficult when you attempt to bond edges or bond pieces in more than one plane. Also keep in mind you don't want the surfaces to get too soft. Practice if possible.
