Hello there.
Yes they can but only to a point. The 4hp would go to a 5hp. A 6hp an 8hp. The 10hp to 15hp. They could not get your 4hp up to anything that would resemble a significant worthwhile upgrade, so I'd suggest your better advised looking for a larger outboard.
Now If only you could find somebody who has
2002 Tohatsu 8hp (electric start)
2006 9.8hp 2 Stroke (black 25 kgs model)
2005 Honda 5hp 4 stroke
1998 Mercury 15hp 2 stroke
2010 Yamaha 20hp 4 stroke
+ others in stock
Peter @ Boatsandoutboards4sale ~
askboatsandoutboards4sale@sky.com ~ 07930 421007