I think that the 4x4 will be strong enough to hold them up without any problems. You may still want to consider putting in a pulley system at the top of the posts for easier harvests though. Also, pullies help with rigs that aren't as tall as yours. You can let the vine reach the top of the post and then use the pulley to lower that vine down to allow more growth upwards. The vine that came from the top can simply go along the ground and will still produce flowers when ready.