Explain for me "promote related products of what they signed-up for".

If you jam products down your subscibers throat they'll likely get tired of you and unsubscribe. That goes for any niche. But, if you're giving them valuable information and recommending products that they may be interested in, you should be good to go.

It depends on how you position it. If you tell them that your four page lead magnet is a complete system, and then try to promote another system, yes, they will be bothered.

If you position your lead magnet as a collection of tips, or an introduction, or such, then promoting a more complete solution IS a product related to what they signed up for.

Later on, promoting things like nutritional supplements, exercise gadgets, specialized courses (such as preparing for a competition) would also be natural extensions from your original lead magnet.

Taking a list of people who signed up for muscle building tips and promoting a business opportunity or IM product would be totally unrelated, and would likely be met with hostility.

I agree with John McAbe's advice. It all goes back to positioning. If you magnet is advertised as a collection of valuable tips and tricks to build muscle, your subscribers shouldn't be offended by your offering them a complete system down the line. After all, four pages isn't going to cover much in that area so they would likely be expecting more information. If offering the system through your follow up series, blend it with more tips and information to ease the "pain" of the offering. If you are giving valuable content across the board, your readers/subscribers will be more open to your offers and less likely to unsubscribe. This strategy also helps mitigate the spam complaint issue.
Again, keep the content relevant to the list segment they subscribed to. Don't go off pushing dog training tips to body building folks as that's not what they signed up to receive. Create and deliver quality, relevant content, provide value, and offer relevant offers as a means to help your list solve their problem and you should be just fine and avoid the spam penalties. If in doubt about your messages, check your autoresponder service. Most have tools built in to help minimize the risk of hitting the spam box.

it does depend on how u do it, do u send them to a thank you page after they opt in,and redirect them to and offer, if so it won't be too much of an issue, as long as its congruent.

No they won't get bothered. What are they to expect? You give them your free bodybuilding course, and then you're going to promote to them a course on growing plants?!?? No.

They wont mind you emailing them about bodybuilding because that's what they signed up for. Just make sure your free ebook is good, your email marketing funnel is good, and the affiliate offer is proven to convert.