Either you're not using a good size well-charged 12V battery, or you don't have a good connection from the starter to the batter, or the brushes inside your starter are worn. Make sure your engine is not flooded either.

If everthing works good, your starter should turn over a .70 4stroke easily. That .70 has got to have some monster compression to give even a std duty starter trouble.

In my experience, 4stroke are actually easier to start than 2stroke. When you wind the prop backwards till you come up to compression (same as on a 2stroke) and engage the starter, the engine will spin about 1-3/4 turns with next to no resistance before encountering the compression stroke. This builds up some nice momentum and helps the starter turn the engine over. 2stroke can only spin free for about 2/3 turn before going up against compression.

I have a Dynatron with a single handle conversion. On 24V, it turns over my Poulan 46cc gasser with ease. Still, I keep a cheapo Tower Hobby HD starter around for my smaller (<1.20) engines to keep the wear and tear down on the Dynatron. There's nothing wrong with the less expensive starters. They don't have easily replaceable brushes (you can still replace them, I've done it before) and they have bushings instead of ball bearings.