You can buy upgrade kits for the DS4. Or you can buy some new 1st stages only.
You wouldn't need to buy a completely new Apeks sidemount reg set, which is also one of the most expensive on the market...
Cheaper options would be HOG or Deep6. Both under 1/2 the cost of Apeks.
Take a look:
The $1200 Budget Sidemount Diver
Kinda like the instructors who don't have sidemount BCDs and teach students it's OK to clip stage-rigged cylinders onto a regular BP&W?
You'd think that when pushing all his students through sidemount courses, he'd have enough money to equip himself in a professional an role-model manner....?
If you are referring to put the DST swivel head on my DS4, it is just another thing of perspective, how you see things and how I see them.
I prefer to have a little not optimal routing vs another leak point and service part.
Divers
Now just to put MY PERSPECTIVE of how I see things, get your potatos chips and a soda or bear because you will take some time to read.
I got my Sidemount kit for 700eu, but with second hand DS4 1st stage with the predecessor of the XTX the ATX 2nd stage ( XTX is just the same but more nicer cosmetics ), I got a nice price for the sidemount system that the Instructor developed, I have not seen noting like it anywhere in quality and good gear, you only can find it in his shop and it quite different than X-deep and a lot more sturdy and quality than X-deep or other brands.
Yes he and some of us can buy the purposely made sidemount regulators, but we DON'T have issues with hose routing, so why spend the money in that, we all spend it in Carbon Bottles instead ( yeah they cost a **** load of money ), it is were you want to spend the money if you have it, it doesn't make since to spend the extra buck to replace something that works fine, unless you have a lot a money and that you don't care, it is a question of perspective.
APEKS is not expensive compared to SP and the ridicules prices of Atomic, all do the same **** and perform the same, a lot of placebo effect at shallow where most diver spend their time.
AL/APEKS are very old brands with very know performance, APEKS makes a lot of regulators for the other Brands, which are sad copies of the APEKS, maybe cheaper, sometimes not, a little cosmetic changes and they called with their name ( very exotic names sometimes ), it is just a sad joke, please open your eyes.
There is nothing new that you can do to a dive 1st stage regulator, it is what it is, all there is is fancy names and a lot of cosmetics and marketing for dumb consumers, there has not been a real break-thru and innovation in the last 15 year or even more, regarding 1st stages.
All there is, is the way you see it on your own perspective.
If I had a lot of money I will buy the FSR from APEKS just because I have to much money, is it really better than a DST or DS4, NO, not really, yeah it have nice cosmetics, nice type of Alloy, a little better routing yes I give you that, but it performs exactly the same, then you hear yeah but it last 50 years because it is a special alloy, please come on, like you will dive that long in your live time, the DS4 you can dive it for probably 20 years or more, and if stops working it way it did before it is just time to buy another one for 3 times less the price of a fancy one,the DS4 had pay back the money years ago if it is that this times come, where you can't service it anymore.
There are a lot silly arguments of this is better than that, this breathe better than those, bla,bla bla, you hardly notice the difference it is a lot of placebo effect, between brands, I'm talking about over-balanced regulators. ( I have noticed differences on non-balanced ones between brands or maybe I encounter my self bad serviced ones vs good serviced ones from different brands)
It is funny, to see people that spend most of there 20 dives a year at 20m max and have very expensive Gear, where actually it is gear that will perform as stated in deep conditions where you really need the good breathing performance numbers, but HEYYYY stop a moment there, that is their money and their perspective.
Just look at the regulators prices in this link
Regulators Regulators 1st stage buy and offers on Scubastore
Look at a price of a DS4 = 149eu , the price of a FSR = 247eu ( good discount ), the price of DST = 255eu
Now guess what !, all three have a Avg WOB of 0.70 joules/liter, in other words they do the exact same ****
Another exercise lets look at Aqua-Lung
AL Core ( aka old Titan more ****** marketing ) = 167eu, the price of a Legend = 329eu, the price of Glacia = 259eu
WOB for the Core is of Avg 0.79 Joules/ liter ( numbers downgraded for marketing so you buy Legends ) the legend and Glacia = Avg 0.65 Joules/liter
You hardly will notice a difference between 0.79 and 0.65 joules/liter
All perform the same, maybe ones a little better in very very cold conditions to minimize a free-flow, but the large majority of divers don't do ice diving, so lest get back to reality.
Same go for the other Brands with expensive stuff and copied stuff.
You go to Malaysia, Indonesia, Philippines, etc, etc and see the pour in the middle of no-where divemasters, with bit up regulators hardly maintained some times non-balanced regulators, do 100's of dives a year, they do way more dives than anybody can think with a 80 Buck regulator that does a little worse than your 500+ regulator, you don't hear that they are dying every month, or do you ??? and they do 100's of more dives than anyone here, so what is actually what, it is just the perspective how you see things, you think a 500+$/EU regulator will be safer than a 80$/EU regulator at 20m depth in tropical waters, everybody is certainly entitled to its own perspective of seeing things the way they do.
I choose the Real Old originals brands if you can call them that, I choose AL or APEKS, but because they have good reputation and they are cheap compared to the other big Brands with exotic names.
Yes I can get me a APEKS copy that is a little cheaper and that perform the same, but for respect of the ORIGINAL if you say it in some way like that, I prefer to buy the real maker, there is truly no better performance of one over the other, they do the same ****, but again that is my perspective, others have other perspectives.
my 2 cents for the day, of like others say, that is my story and I'm sticking to it.