@Jan You apparently have internet, so I recommend doing a google or YouTube search for “insulin pen” and “insulin pen half unit” to find references so you can evaluate for yourself. My experience is with only one brand. That said, here’s my commentary:
I can’t speak to pros/cons of 4 vs 8mm. I’ve only used 4s. Differences are obviously depth, potential discomfort, and potential for going IM (intramuscular) vs sub-c (I don’t think 8mm would go IM, but others here can advise with direct knowledge).
Ref pens: They exist for about every insulin type. Most are self-contained plastic throw away units with 3ml each, boxes of five (5) pens. They use screw/twist on disposable needles that seem fairly standardized connectors in various lengths/gauges and bought separately from pens; I like BDs, others like other brands, costs vary. Most are U100, but other strengths exist. Some are “smart” like Inpen and internally record/link to apps via Bluetooth for logging when and how much; InPens are good for one year, then you buy another, cost is often covered by insurance, but you can buy for $35. Some pens, like Novolog Echo, have 1/2 unit dosing (others exist, just google for 1/2 unit insulin pens). The Echo (How to use NovoPen Echo® - Quick Guide - YouTube), while a pen, uses cartridges of insulin, instead of being self contained; the exterior appears to be metal cap and body with plastic cartridge holder inside that twists off the metal pen body that holds the plunger/dosing mechanism. To avoid confusion, the InPen and Echo (probably others that take cartridges) come in colors to differentiate short and long acting insulins. Both have similar twist dialing for dose, except you dial in half units instead of full units.
If you’re using Lantus with 10/11 units once a day, splitting your dose is pretty easy. I’m no doc and you should discuss with him/her before or after as you’re comfortable with. I did it on my own, tried splitting the dose that seemed to cause early morning lows in half (instead of 12 at night, I took 6 one night at bedtime, then another 6 the next morning, my times were slightly asymmetrical, not strictly 12 hours apart). I noticed I ran a bit higher at night and between meals, so I upped the morning dose 1 unit after a couple of days; I still had the problem at night, so upped the evening dose 1 unit for a total of 14 per day. NOTE: If you do this yourself, I recommend giving yourself 2-3 days for each change to let things “settle” in and ensure it’s working/not working, alternately talk about with your doc. If it works for you, great! If it doesn’t switch back, but watch for a low that first day as you’ll have ≈50% more in your system for that first higher dose. I’ve heard from a few others having similar experiences as mine, so you could find you will need a slightly increased Lantus dose total.
I agree with learning, I’ve learned more about being/treating T1 diabetes (dosing for carbs, dosing for fat/protein, pumps, CGMs, etc.) from JuiceBox, Taming the Dragon, FUD, TCOYD, T1Exchange, etc., than I ever have in 15 minute appts at my doc or Endo (not dis’ing Dr’s, just the time our system allows them to dedicate per person and the lack of a team approach, i.e. including a CDE, dietician, DNP). And I’m still learning from all comers: old, young, long-time T1s, short-time T1s, and a bunch of non-T1 parents of T1s!
). I found the BD Nano 2nd Gen needles were much more comfortable than Droplet needles (only two I tried, Wegman’s only had the BDs for my first set, military pharmacy only had Droplets. I like the BDs so much, I got the doc to write the script for them only!).
This one I’ll have to think about. What keeps the plunger from going any farther? It sure looks like there is insulin in that headspace in 
, but I just want to understand it. 
