No. Impedance has nothing to do with loudness. If a particular 4 ohm speaker is significantly less efficient than a given 8 ohm speaker, it could play less loudly, even if it is supplied with more wattage input.

Efficiency is the rating which determines how loud a speaker will play for a given wattage input. In addition, there is a very definite limit to how loud a particular speaker will play before destruction, and this is determined by characteristics of it's design, usually involving more cost for more robust parts.

Efficiency is rated as a certain number of decibels sound pressure when a speaker is supplied with one watt of power, at a distance of 1 meter. A typical rating might be 89db for 1 watt. A difference of 3dB equals an efficiency difference of a factor of 2. A speaker with a rating of 89db will play twice as loud (3db louder) as a speaker rated at 86db when supplied with 1 watt, regardless of it's impedance.

It's worth noting that the ear does not hear 3db as subjectively "twice as loud" - as a matter of fact, 3db is just audible as a loudness increase (or decrease). To make a subjective difference of being "twice as loud", the increase must be around 10db.