We have 4x8 foot beds too.

I would broadly group plants into four categories:

• Things that stay low to the ground and don't spread much. Examples include carrots, onions, lettuce, etc. You can plant these fairly densely
• Things that climb (climbing beans, cucumbers, beans, squash, melons, peas, etc) or at least grow mostly vertically like indeterminate tomatoes. You can plant these somewhat densely. We tend to have one bed that has an 8' row of carrots, an 8' row of climbing peas and pole beans , a double/triple row of onions, an 8' row of indeterminate tomatoes spaced every 20" or so in a Florida weave, and another 8' row of carrots
• Things that are more bush shape, but aren't that unwieldy. Easy examples are bell peppers, broccoli/cauliflower, cabbage, and egg plant. We will plant eight of these per 4x8 bed
• Things that sprawl. These get their own half a bed, which means we don't grow them all that often. Examples include non-climbing squash/melon, non-climbing cucumbers, etc

There is a little opportunity for hybridization by putting something that sprawls in a bed with something that climbs where the climbing structure is on one of the sides of the bed. If you want to try a busy bed, I would avoid things that sprawl.