Hi iRFNA,
You can read my comments about that particular review and their use of 1.5 lb polyfoam in post #108 here and the posts it links to.
4 lb memory foam is a medium density memory foam that for most people would have reasonable durability and wouldn’t be a weak link in a mattress unless they were in a higher weight range (around 200 lbs or higher) at which point I would reduce or minimize the use of memory foam under 5 lbs. With your lower weight lower density foams would be more durable than they would for higher weight ranges.
You can read more about the different factors and variables that can affect the durability and useful life of a mattress in post #4 here and the other posts it links to.
This would be very much an apples to oranges comparison because memory foam and latex are as close to “opposites” as it’s possible to get in two different foam materials (see post #2 here). I would suggest some local testing on each type of foam so you know which one you tend to prefer although both of these are members of this site which means that I believe they compete well with the best in the industry and are both good quality/value choices.
When you are considering an online purchase then the exchange or return policy (including any costs involved in returning or exchanging either individual layers or the complete mattress) can be an important part of your personal value equation and is certainly something that I would factor in to the value of a purchase.
Generally no as long as you are using a suitable foundation that meets their warranty criteria but it’s always a good idea to verify this with the manufacturer or retailer themselves.
Just in case you haven’t read it yet … I would also suggest reading the tutorial post here which includes the more important information, steps, and guidelines that can help you make the best possible choice.
Phoenix