MARC does not run on weekends or most holidays (yet; this will change). Amtrak does run on 4 July, but will be expensive as it's a holiday. WMATA's buses, including the B30 ($6), operate on a Saturday schedule on the 4th, which on that route means business as usual since M-Sa schedules are similar. The bus runs pretty much on schedule, in my experience. Then you'll transfer to the Green Line, which is $4.50 to downtown DC.

Perhaps my time's less valuable, but I'd fly out of BWI to save $50, or if it meant the difference between a saver/standard award.