OP, so really your house is a 3 bed house in terms of everything except an additional bedroom above the garage. That makes it all rather confusing for buyers - they see it advertised as 4 beds (which it is) and then expect not just 4 beds but a full 4 bed house....and that's what they are looking for. A usual feature of a 4 bed is a downstairs loo, probably a bit more living space than you have too, possibly a larger kitchen - unless you've extended in these other areas too.
So all this extending and converting which goes on,morgen does add one extra room to a house, but doesn't make it into a TRUE 4 or 5 bed or whatever, because the overall footprint of the house doesn't reflect what is associated with a house with that many bedrooms. It can muddy the water regarding price but also interns of expectations from buyers.
So I assume your house isnroeiced somewhere between a 3 and 4 bed house. The viewers mig see it as a cheaper 4 bed, but haven't reconciled to a lack of 4 bed features because hey are in the market for a 4 not 3 bed with an extra room. Unless yours is substantially cheaper it may struggle to sell and actually things like a downstairs loo are often part of upgrading from 3 to 4 beds.
Great you could get one in. If it's proving a stumbling block, pay someone to do the whole job ASAP and then you'll know you've removed that barrier.
Similar barriers to sale I've seen with extended houses are those which have had an extra bedroom or 2 added in the loft, so have become 4 or 5 bed roomed, but still only have 1 smallish living room or a smallish living room and a kitchen you can't eat in, or have a very small garden, or lack off street parking.......they overall don't feel like a 4 or 5 bed house and won't fetch the price of one.
When extending is important to keep the house in proportion, if re-sale is a factor - and it will be for everyone at some point.
So I'd say a 4 bed needs to have at least 2 family spaces - that can be a dining room and living room, or a living room and a large kitchen which also has a sofa in it - just 1 isn't enough. It also needs at least 2 loos - 1 might be Ensuite, but 2 loos which are usable for everyone living in the house is good, plus a downstairs loo for guests is very usual in a 4 bed. Most people also expect a reasonable siz d garden given 4 bed is a family home and would also expect off road parking unless the house is very old or in a very urban setting.
Buyers have expectations about both what they should get for a price but also what should be included in a particular type or size of house (usually measured in terms of bedroom numbers) and anyone offering less than what is considered usual or normal for that house can struggle to sell,neven at a slightly lower price. Sometimes they have to price at the bedroom size below and even sometimes market it as the smaller bedroom number,must to attract viewers through the door at that price and expectation level.